Attribute the cost before editing. Run the profiler from the table below on a local reproduction or replay of the real workload, and name the function, allocation site, or wait that dominates. Profiling a production process needs the user's approval (overhead, customer data in samples). Optimizing code the profile does not show moves no metric and adds risk.
Write the equivalence test first. Keep the old implementation as a baseline function and compare old and new on empty, boundary, error, aliasing, and ordering inputs. Faster code that changes results, error text, ordering, or nil versus empty is a regression.
Benchmark release builds only: -O2, cargo build --release, Go default, dotnet build -c Release, swift build -c release, a JIT that has warmed up. Debug and unoptimized numbers point at different hot spots.
Make the benchmark consume its result. Dead-code elimination deletes a loop whose output is unused, and constant inputs let the compiler fold the work. Use the harness sink (Blackhole, black_box, b.Loop, DoNotOptimize, BenchmarkDotNet return values) and inputs the compiler cannot see through.
Warm up before timing on JIT runtimes (JVM, .NET, V8, PyPy) and discard the warmup. Cold-start cost is a separate metric: measure it only when startup is the complaint.
Repeat and compare distributions, not single numbers. Run at least 10 samples each for baseline and candidate on the same machine, inputs, and power state, and use the comparison tool of the harness (benchstat, pyperf compare_to, criterion, JMH error bars). A difference inside the noise is no difference: revert it. Do not rerun until a result becomes significant.
Change one thing per measurement so the result is attributable. Revert a change whose metric shows no movement; a plausible rewrite often measures as nothing.
Invalid speedups: less work (skipped validation, cached result, smaller input), a benchmark that measures setup, and a microbenchmark win that vanishes in the application. Re-run the application-level workload after the last change and report both numbers.
Check the target, not the laptop. Record compiler and runtime versions, flags, CPU, and runtime settings (GOMAXPROCS, heap size, GC mode). Timings do not transfer between machines; allocation counts usually do.
Build-wide switches (LTO, PGO, -march=native, GC settings) change the whole binary. -march=native produces binaries that crash on older CPUs in the fleet. Measure at application level, state the deployment consequence, and stop for the user's approval before changing the shipped build configuration.
Keep public API and thread-safety contracts. Pooling, buffer reuse, and caching introduce aliasing and shared-state bugs; say what the caller may now not do.

Workflow

Reproduce the workload and pick the metric: CPU time, tail latency, throughput, allocations per operation, live heap, or RSS.
Profile with the tool for the language and read its reference for the traps of that stack.
Write the baseline and the oracle test.
Apply one change, run the oracle, then the benchmark (at least 10 samples each), then the comparison tool.
Keep or revert. Repeat from the new profile, since the top of a profile changes after each fix.
Report the profile that justified the change, the oracle result, before and after with their spread, and each check or measurement not run, by name.

Leave tsc and type-check speed to $optimize-tsc-builds, and refactors for readability to the refactor-code-smells skill.

Languages

Language	Profile and benchmark	Reference
C	`perf record` (Linux) or WPA with ETW (Windows), `valgrind --tool=callgrind` (Linux), sanitizers, `-fopt-info` or `-Rpass`	c
C++	`perf` and `heaptrack` (Linux) or WPA with ETW (Windows), Google Benchmark, `objdump -d -C` (`dumpbin /disasm` for MSVC)	cpp
C#	BenchmarkDotNet, `dotnet-counters`, `dotnet-trace`	csharp
Go	`go test -bench -benchmem -count 10`, `benchstat`, `pprof`	go
Java	JMH, JFR, `-Xlog:gc`, `-XX:+PrintCompilation`	java
JavaScript	`node --cpu-prof`, `--trace-deopt`, DevTools, `mitata`	javascript
Kotlin	JMH with `kotlinx-benchmark`, JFR, `javap -c`	kotlin
Python	`pyperf`, `cProfile`, `tracemalloc`, `-X importtime`	python
Rust	Criterion or `divan`, `samply`, `cargo asm`, DHAT	rust
Scala	JMH via `sbt-jmh`, JFR, `javap -c`	scala
Swift	`package-benchmark`, Instruments, `-emit-sil`	swift

Scripts

python3 scripts/compare_benchmarks.py BASELINE.csv CANDIDATE.csv --keys COL... --max-regression-percent N compares two complete BenchmarkDotNet-style CSV tables row by row and fails on a duration regression. Every column must be a key, the metric, or ignored, so it never guesses row identity. Exit 0 within the threshold, 1 regression, 2 invalid input. Run it with --help. On Windows, use py -3 for python3. For other harnesses use their own comparison tool.

References

Read the language reference from the table before applying a change. Each lists mistakes that look like optimizations and their fixes. Read jvm as well when the code runs on the JVM and the cost is JIT warmup, GC, or the JMH harness.

optimize-runtime-performance

Optimize Runtime Performance

Rules