As an update to my 2012 post on OpenCart being slow with many categories, there have been many discussions, 2 , 3. There have been a few solutions offered, all of which have been rejected by the OpenCart developers for some strange reason.
i don’t believe this. i think you have mis understood something.
you shoudl be using indexes and not making new primary keys!
@ChrisAtomix has released OpenCart Turbo. A run-once script that speeds up your OpenCart store, applying the fixes I discussed in 2012 and few other speed up changes.
I’d recommend running this on any OC site, not just sites that are slow right now.