"This page feels slow" is a vague bug report. Before changing any code, it helps to have a fixed way...
Rails slowness stems from N+1 in views; a page with 48 products had 49 render queries instead of O(1) after preloading associations. N+1 scales with traffic and data; layer-by-layer debugging via rack-mini-profiler, bullet, and EXPLAIN prevents production performance debt.
"This page feels slow" is a vague bug report. Before changing any code, it helps to
have a fixed way to locate where the time goes. A Rails request passes through a few
predictable layers, and each layer has its own tools and its own typical failure mode.
Here is the mental model I use, and a real example of walking it end to end.
The layers of a request
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