I downloaded a bunch of brushes, and there's a few ones at default that are really, really laggy.
Is there a way to decrease this lag?

Only by decreasing the spacing, or using autospacing, but big brushes will always lag more than small ones.

Select all and delete, the line tool, the bucket tool, etc are also all slow, I just noticed :/.
Some of the smaller brushes are slow too. changing opengl settings doesn't help. My canvas size is an A4 paper (in Centimeters) with 300 dpi.
It's annoying because mypaint, corel painter, photoshop/flash/illustrator, and gimp don't do this, but I think I like Krita the most.
specs:
Windows 7 64 bit (running the 64 bit version)
8 GB Ram
Nvidia Geforce GTX 560 TI
Intel Core I5-750

You may have the NVidea slowdown as experienced here: viewtopic.php?f=281&t=125170

We're still trying to figure out why this happens.

TheraHedwig wrote:You may have the NVidea slowdown as experienced here: viewtopic.php?f=281&t=125170

We're still trying to figure out why this happens.

Updated my drivers and rebooted, still the same. Weird, guess I'm just one of the unlucky ones when it comes to setup :/
Technical issues I'm having aside, I'm really loving this program, it's really really good, and free