Recently I've discovered a quite big perfomance problem when running INFORMATION_SCHEMA queries on mysql.
In particular I found that on some systems (some mysql versions) a query like
SELECT * FROM INFORMATION_SCHEMA.COLUMNS WHERE TABLE_SCHEMA = 'db_name' AND TABLE_NAME = 'db_table'
can take up to 0.1s to run!
Clearly such time interval is not acceptable, and I think it happens only with some versions of mysql because I've run some tests on my local machine and on the server where this site is running obtaining a result of 0.0007 seconds, we are talking about a difference of two orders of magnitude!
I've not investigated more about which mysql versions are affected and if there are any patches, I ...