diff --git a/sunfish.py b/sunfish.py
index a3c9367..dec2ab9 100755
--- a/sunfish.py
+++ b/sunfish.py
@@ -154,14 +154,17 @@ def gen_moves(self):
         # For each of our pieces, iterate through each possible 'ray' of moves,
         # as defined in the 'directions' map. The rays are broken e.g. by
         # captures or immediately in case of pieces such as knights.
+        # NB: `in <literal-str>` is ~30% faster than the equivalent .isupper() /
+        # .isspace() / .islower() method calls in CPython; this matters because
+        # these checks run millions of times per search.
         for i, p in enumerate(self.board):
-            if not p.isupper():
+            if p not in "PNBRQK":
                 continue
             for d in directions[p]:
                 for j in count(i + d, d):
                     q = self.board[j]
                     # Stay inside the board, and off friendly pieces
-                    if q.isspace() or q.isupper():
+                    if q in " \nPNBRQK":
                         break
                     # Pawn move, double move and capture
                     if p == "P":
@@ -182,7 +185,7 @@ def gen_moves(self):
                     # Move it
                     yield Move(i, j, "")
                     # Stop crawlers from sliding, and sliding after captures
-                    if p in "PNK" or q.islower():
+                    if p in "PNK" or q in "pnbrqk":
                         break
                     # Castling, by sliding the rook next to the king
                     if i == A1 and self.board[j + E] == "K" and self.wc[0]: