[tracker1]

I've seen that happen a lot as JSON(XHR) and ORMs started to become more common... certain queries would return way too much data from related (auto-fetched) records and it was just slow AF on remote computers.

Another common one is just poor query performance from a database. Lack of appropriate indexes, or other relatively easy optimizations.

Similarly, finding a method of caching that's just bad (in-memory database, with sql queries instead of a dictionary). Isn't so bad for one call, very bad when a a given request (login) makes over 200 calls to this cache for configuration settings. It wasn't a problem per request but in aggregate.