[development] Caching, caching, caching...

[Moshe Weitzman]

> @Dries - did you have the devel.module query log enabled when you did 
> your analysis? That log calls debug_backtrace() for every query and 
> could conceivably slow things down.

let me revise that - if you turned on the query log and subsequently 
disabled devel.module, you would still pay the performance penalty because 
the variable_get('dev_query', 0) will still be true.

to fix this, we should introduce an uninstall hook and advise module authors 
when they should and should not use it (this is an example of *should use it*).