Chromium Debugger Found to Throttle fetch() Uploads 20x, Skewing Performance Tests

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A developer at Coffer, a browser-side encrypted file storage service, spent considerable time investigating why upload speeds ran at roughly one-eighth of available gigabit bandwidth while downloads nearly saturated the link. Three successive fixes — reducing chunk count, adding encryption-transfer pipelining, and exploring concurrency — each appeared promising but failed to produce meaningful or reproducible improvements. Careful component-level benchmarking revealed that client-side encryption accounted for only about 2% of total chunk time, making earlier gains statistically indistinguishable from noise. The root cause was ultimately traced to the measurement environment itself: attaching a debugger to Chromium throttled fetch() uploads by approximately 20 times, meaning the benchmark was actively causing the performance problem it was trying to measure. The case highlights how debugging tools can silently distort network performance data, leading engineers to chase phantom bottlenecks across multiple system layers.

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