diff --git a/avro-data/src/main/java/io/confluent/connect/avro/AvroData.java b/avro-data/src/main/java/io/confluent/connect/avro/AvroData.java
index a5f04ef5cd5..50f3c7ae75c 100644
--- a/avro-data/src/main/java/io/confluent/connect/avro/AvroData.java
+++ b/avro-data/src/main/java/io/confluent/connect/avro/AvroData.java
@@ -607,7 +607,10 @@ avroSchema, maybeWrapSchemaless(schema, converted, ANYTHING_SCHEMA_MAP_FIELD),
           //This handles the inverting of a union which is held as a struct, where each field is
           // one of the union types.
           if (isUnionSchema(schema)) {
-            for (Field field : schema.fields()) {
+            List<Field> fields = schema.fields();
+            int numFields = fields.size();
+            for (int i = 0; i < numFields; i++) {
+              Field field = fields.get(i);
               Object object = ignoreDefaultForNullables
                   ? struct.getWithoutDefault(field.name()) : struct.get(field);
               if (object != null) {
@@ -625,7 +628,10 @@ avroSchema, maybeWrapSchemaless(schema, converted, ANYTHING_SCHEMA_MAP_FIELD),
             org.apache.avro.Schema underlyingAvroSchema = avroSchemaForUnderlyingTypeIfOptional(
                 schema, avroSchema, scrubInvalidNames);
             GenericRecordBuilder convertedBuilder = new GenericRecordBuilder(underlyingAvroSchema);
-            for (Field field : schema.fields()) {
+            List<Field> fields = schema.fields();
+            int numFields = fields.size();
+            for (int i = 0; i < numFields; i++) {
+              Field field = fields.get(i);
               String fieldName = scrubName(field.name(), scrubInvalidNames);
               org.apache.avro.Schema.Field theField = underlyingAvroSchema.getField(fieldName);
               org.apache.avro.Schema fieldAvroSchema = theField.schema();
@@ -1648,7 +1654,10 @@ private Object toConnectData(Schema schema, Object value, ToConnectContext toCon
                   valueRecord.getSchema(), true, null, null, toConnectContext);
             }
             int index = 0;
-            for (Field field : schema.fields()) {
+            List<Field> fields = schema.fields();
+            int numFields = fields.size();
+            for (int i = 0; i < numFields; i++) {
+              Field field = fields.get(i);
               Schema fieldSchema = field.schema();
               if (isInstanceOfAvroSchemaTypeForSimpleSchema(fieldSchema, value, index)
                   || (valueRecordSchema != null && schemaEquals(valueRecordSchema, fieldSchema))) {
@@ -1666,7 +1675,10 @@ private Object toConnectData(Schema schema, Object value, ToConnectContext toCon
             // Default values from Avro are returned as Map
             Map<CharSequence, Object> original = (Map<CharSequence, Object>) value;
             Struct result = new Struct(schema);
-            for (Field field : schema.fields()) {
+            List<Field> fields = schema.fields();
+            int numFields = fields.size();
+            for (int i = 0; i < numFields; i++) {
+              Field field = fields.get(i);
               String fieldName = scrubName(field.name());
               Object convertedFieldValue = toConnectData(field.schema(),
                   original.getOrDefault(fieldName, field.schema().defaultValue()),
@@ -1677,7 +1689,10 @@ private Object toConnectData(Schema schema, Object value, ToConnectContext toCon
           } else {
             IndexedRecord original = (IndexedRecord) value;
             Struct result = new Struct(schema);
-            for (Field field : schema.fields()) {
+            List<Field> fields = schema.fields();
+            int numFields = fields.size();
+            for (int i = 0; i < numFields; i++) {
+              Field field = fields.get(i);
               String fieldName = scrubName(field.name());
               int avroFieldIndex = original.getSchema().getField(fieldName).pos();
               Object convertedFieldValue =
diff --git a/json-schema-converter/src/main/java/io/confluent/connect/json/JsonSchemaData.java b/json-schema-converter/src/main/java/io/confluent/connect/json/JsonSchemaData.java
index f1458bd0b5b..a0e68a13f81 100644
--- a/json-schema-converter/src/main/java/io/confluent/connect/json/JsonSchemaData.java
+++ b/json-schema-converter/src/main/java/io/confluent/connect/json/JsonSchemaData.java
@@ -545,7 +545,10 @@ public JsonNode fromConnectData(Schema schema, Object logicalValue) {
           //This handles the inverting of a union which is held as a struct, where each field is
           // one of the union types.
           if (isUnionSchema(schema)) {
-            for (Field field : schema.fields()) {
+            List<Field> fields = schema.fields();
+            int numFields = fields.size();
+            for (int i = 0; i < numFields; i++) {
+              Field field = fields.get(i);
               Object object = config.ignoreDefaultForNullables()
                   ? struct.getWithoutDefault(field.name()) : struct.get(field);
               if (object != null) {
@@ -555,7 +558,10 @@ public JsonNode fromConnectData(Schema schema, Object logicalValue) {
             return fromConnectData(schema, null);
           } else {
             ObjectNode obj = JSON_NODE_FACTORY.objectNode();
-            for (Field field : schema.fields()) {
+            List<Field> fields = schema.fields();
+            int numFields = fields.size();
+            for (int i = 0; i < numFields; i++) {
+              Field field = fields.get(i);
               Object fieldValue = config.ignoreDefaultForNullables()
                   ? struct.getWithoutDefault(field.name()) : struct.get(field);
               JsonNode jsonNode = fromConnectData(field.schema(), fieldValue);
diff --git a/protobuf-converter/src/main/java/io/confluent/connect/protobuf/ProtobufData.java b/protobuf-converter/src/main/java/io/confluent/connect/protobuf/ProtobufData.java
index 3eda2e5e611..7f46b03841d 100644
--- a/protobuf-converter/src/main/java/io/confluent/connect/protobuf/ProtobufData.java
+++ b/protobuf-converter/src/main/java/io/confluent/connect/protobuf/ProtobufData.java
@@ -460,7 +460,7 @@ private Object fromConnectData(
           if (listValue.isEmpty()) {
             return null;
           }
-          List<Object> newListValue = new ArrayList<>();
+          List<Object> newListValue = new ArrayList<>(listValue.size());
           for (Object o : listValue) {
             newListValue.add(fromConnectData(ctx, schema.valueSchema(), scope, o, protobufSchema));
           }
@@ -468,7 +468,7 @@ private Object fromConnectData(
         case MAP:
           final Map<?, ?> mapValue = (Map<?, ?>) value;
           String scopedMapName = ((Descriptor) ctx).getFullName();
-          List<Message> newMapValue = new ArrayList<>();
+          List<Message> newMapValue = new ArrayList<>(mapValue.size());
           for (Map.Entry<?, ?> mapEntry : mapValue.entrySet()) {
             DynamicMessage.Builder mapBuilder = protobufSchema.newMessageBuilder(scopedMapName);
             if (mapBuilder == null) {
@@ -508,7 +508,10 @@ private Object fromConnectData(
           //This handles the inverting of a union which is held as a struct, where each field is
           // one of the union types.
           if (isUnionSchema(schema)) {
-            for (Field field : schema.fields()) {
+            List<Field> fields = schema.fields();
+            int numFields = fields.size();
+            for (int i = 0; i < numFields; i++) {
+              Field field = fields.get(i);
               Object object = ignoreDefaultForNullables
                   ? struct.getWithoutDefault(field.name()) : struct.get(field);
               if (object != null) {
@@ -527,7 +530,10 @@ private Object fromConnectData(
             if (messageBuilder == null) {
               throw new DataException("Invalid message name: " + scopedStructName);
             }
-            for (Field field : schema.fields()) {
+            List<Field> fields = schema.fields();
+            int numFields = fields.size();
+            for (int i = 0; i < numFields; i++) {
+              Field field = fields.get(i);
               String fieldName = scrubName(field.name());
               Object fieldCtx = getFieldType(ctx, fieldName);
               Object connectFieldVal = ignoreDefaultForNullables
@@ -1224,7 +1230,10 @@ protected Object toConnectData(Schema schema, Object value) {
           final Struct struct = new Struct(schema.schema());
           final Descriptor descriptor = message.getDescriptorForType();
 
-          for (OneofDescriptor oneOfDescriptor : descriptor.getRealOneofs()) {
+          List<OneofDescriptor> oneOfDescriptors = descriptor.getRealOneofs();
+          int numRealOneOfs = oneOfDescriptors.size();
+          for (int i = 0; i < numRealOneOfs; i++) {
+            OneofDescriptor oneOfDescriptor = oneOfDescriptors.get(i);
             if (message.hasOneof(oneOfDescriptor)) {
               FieldDescriptor fieldDescriptor = message.getOneofFieldDescriptor(oneOfDescriptor);
               Object obj = message.getField(fieldDescriptor);
@@ -1238,7 +1247,10 @@ protected Object toConnectData(Schema schema, Object value) {
             }
           }
 
-          for (FieldDescriptor fieldDescriptor : descriptor.getFields()) {
+          List<FieldDescriptor> fields = descriptor.getFields();
+          int numFields = fields.size();
+          for (int i = 0; i < numFields; i++) {
+            FieldDescriptor fieldDescriptor = fields.get(i);
             OneofDescriptor oneOfDescriptor = fieldDescriptor.getRealContainingOneof();
             if (oneOfDescriptor != null) {
               // Already added field as oneof