How do you troubleshoot slow ETS lookups on a heavily-used table?

A "fast" O(1) ETS set/bag lookup can still degrade under real load for a handful of specific, diagnosable reasons rather than randomly — the first step is confirming which of these actually applies before reaching for a fix.

ets:info(Tab, memory),          %% table size, in words
ets:info(Tab, size),            %% row count
ets:info(Tab, [read_concurrency, write_concurrency]).

1. Lock contention — many concurrent writers without write_concurrency enabled serialize on the same lock; check ets:info/2 for the current setting and enable it if writes are frequent and concurrent.
2. Oversized keys/values copied on every read — a lookup still copies the matched row into the caller; storing large blobs directly as values means every read pays that copy cost, so storing a reference (e.g. a binary handle) instead can help.
3. Using ets:match/2 where ets:select/2 with a compiled match spec would filter more efficiently.
4. An ordered_set used where a plain set would do — ordered tables cost more per operation (tree-based) than hash-based ones when strict ordering isn't actually needed.

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