Strengthen passthrough proxy section with L7 and contention insights

- Reframe the takeaway: the proxy boils the latency-sensitive path to
  near-TCP-stack overhead while operating at Layer 7 — that's the win
- Add paragraph explaining why overhead holds across 10/100 topics: the
  proxy doesn't contend between topics (unlike a broker which juggles
  disk I/O, partition leaders, and replication); the connection sweep
  validates linear throughput scaling

Assisted-by: Claude Sonnet 4.6 <noreply@anthropic.com>
Signed-off-by: Sam Barker <sam@quadrocket.co.uk>

Author: SamBarker

_posts/2026-05-21-benchmarking-the-proxy.md

@@ -67,8 +67,11 @@ Good news first. The proxy itself — with no filter chain, just routing traffic
 
 **The headline: ~0.2 ms additional average publish latency. Throughput is unaffected.**
 
-What did I take away from this entirely unsurprising result? Not much, honestly — without filters the proxy is little more than a couple of hops through the TCP stack, but we now have data rather than a hunch.      
-The end-to-end (E2E) p99 figure is dominated by the Kafka consumer fetch timeouts, as it should be. That said, it is reassuring to have a sub-ms impact on the p99.
+What did I take away from this entirely unsurprising result? Not much, honestly — without filters the proxy boils the latency-sensitive path down to little more than a couple of hops through the TCP stack. We replaced a hunch with data. The remarkable part: the proxy is doing this at Layer 7.
+
+The overhead holding across 10 and 100 topics makes sense for the same reason: the proxy doesn't contend between topics. A Kafka broker juggles disk I/O, partition leaders, and replication across everything it manages; the proxy treats each connection independently. Topics don't contend for shared resources: throughput scales linearly across them, and the connection sweep validates it.
+
+The end-to-end p99 figure is dominated by Kafka consumer fetch timeouts, as it should be. That said, it is reassuring to have a sub-ms impact on the p99.
 
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