Don't use SQL_CALC_FOUND ROWS on simple queries, when you have to run a simple query on one or two (with join) tables with a simple WHERE. Especialy if tables are big. I found out that single SELECT COUNT(id) FROM table is far more faster when query has simple WHERE conditions. I use SQL_CALC_FOUND_ROWS only with a rather complex queries - that SQL_CALC_FOUND_ROWS doesn't affect query execution at all (well, it affects ofcourse, but affect is so small, you can't see it).
