Profilers, flamegraphs, benchmarks and caches: find the slow part first, then fix only that.
Use Tokio runtime and task metrics to see worker busy time, scheduling delay, queue depth and slow polls. Export them, read the numbers that matter and tell blocking code from a genuine lack of capacity.
The average hides the slow requests users remember. Record latency in histograms, read p50, p95 and p99, learn why tails grow with fan-out, and what typically causes them in Rust services.
Serve the cached value immediately, even if slightly old, and refresh it in the background. Learn the idea, the HTTP Cache-Control directives and an in-process implementation with a freshness window and a grace window.
When a hot cache entry expires, a hundred requests recompute it at once. Coalesce concurrent misses so one loads and the rest wait, with moka get_with, a hand-rolled version and a note on stale-while-revalidate.
Take a slow SQLx query, run EXPLAIN ANALYZE, and read the plan: sequential scan versus index scan, row estimates, sort and join nodes. Add the right index and confirm it is used, including partial and composite ones.
Find where a Rust program allocates and how much it holds. Use /usr/bin/time for peak RSS, heaptrack for Linux allocation timelines, the dhat crate for portable heap profiles and a counting allocator for tests.
Swap the global allocator with one line. Compare the system allocator, jemalloc, mimalloc and snmalloc on fragmentation, thread scaling, memory return and portability, and measure before switching.
Arrange data for how it is read, not how it is modeled. Compare array of structs with struct of arrays, see why hot loops get faster with contiguous fields, and when the change is not worth it.
Read a file line by line with BufReader and reuse a buffer, or map it into memory with memmap2. Compare the options, the unsafe contract of mmap, and when reading in chunks is the safer and equally fast choice.
Use hyperfine to compare your CLI with grep, awk or a pipeline: warmup runs, prepare commands for cache control, parameter sweeps and export. Avoid the traps that make comparisons unfair.
Rust's regex crate guarantees linear-time matching, so there is no catastrophic backtracking. The real costs are compiling in a loop, unanchored scans and capture groups. Learn what to cache and when to use regex-lite.
Swap iter for par_iter and Rayon splits the work across a thread pool. See the speedup conditions, the traits it needs and when it makes things slower.
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