I have the ame issue as rdanhoffer, but over Linux.
The distro is installed on a Virtual Machine creaated with VMware Workstation.
Specs
OS: Linux Mint KDE 17.2 Rafaella x86
RAM: 2GB
VGA: VMware adapter, 256 MB, 3d enabled, aplied all tweaks to even get Oxigen to work with OpenGL, acceleration works
HDD: 80 Gigs
Krita version: 2.8.5 (I haven't been offered an upgrade to 2.9 branch yet...)

Basically, Krita starts great, and all operations go well, exept that, without OpenGL enabled, you can draw freely, but canvas operations are slow (Zoom or Rotate), if you enable OpenGL, the canvas operations work at lightning speed, but drawing on the canvas gets really slow, tried diabling all smoothigh setting on the OpenGL Tab, but it's still low...

Maybe this has something to do with OpenGL code version you use on Krita? I've heard versions prior to 2.0 are slow as heck, whereas 2.0 and above add preformance improvements via new instructions (?)

The Host system I run thi VM is an Acer Aspire 4535, 4 GB RAM, AMD Athlon QL-65 2.1 Ghz, 500 GB HDD 7200 RPM, ATI Radeon HD 3200 (256 MB), as a note, when I run the Linux machine, I disable all visual effects on Windows 7 x64 so I can allocate more resources to Linux Mint.

If you're getting Krita through steam, you can select 2.9 in the steam settings thing, it'll be the desktop branch. Running Krita in a VM is always going to be problematic, especially when it comes to opengl.

Well, hopefully I got assistance and I could install 2.9... nope, I don't use steam, just Synaptic and the Software Manager...
I've tested 2.9.7 though, UI is way better now, and OpenGL eems to work better, but, even if this is a VM, I'm getting unexpected slowdowns while on 8 bit mode, if you choose 16 or 32 bit, paint speed increases, thi happens with or without OpenGL enabled (but better to enable it, becaue it makes canvas operations way faster).

It's funny that heavier and more demanding documents (16 or 32 bit) work better than 8 bit... Maybe has something to do with the way the calculations are done for brushes and their drawing operations... dunno... just an idea, the only thing I can code to save my life (and still struggle) is HTML and CSS XD...

I can confirm this happens in Windows 7 (or 8 or 10) and Linux Mint KDE.

But, anyway, congratulations, magnificent work on this great alternative, it's really a relief to know such a program exist, so if W7 "dies" (2020) I can switch to KDE and keep doing stuff

If 16f or 32f images perform better it's a graphics card/driver issue. With some combinations, the card only handles 16f or 32f numbers so 8 bit or 16 bit integer has to go through a slow conversion step when uploading the rendered image to the card's memory.

I guess it depends on the architecture you are using for Krita, because, for example, compared to Photoshop CS6 x64, there are performance penalties, specially with OpenGL (Photoshop uses the same to render canvas, brushes and effects) and the weirdest thing is that 8 bit images render perfectly in Photoshop, while Krita struggles with them. On the other hand, there is software like MyPaint, that can render brushes and canvas operations faster, but I'm in doubt if they use OpenGL or access to GPU in any moment, so maybe is not an example for this particular issue. So this discards anything with drivers or GPUs not being able to handle this...

Not comparing, as I said, Krita is a great alternative, and maybe, using PS as a refference you can somewhat address the issues with 8 bit images rendering, and brush engine in general. No easy task though, this is why Graphics software are more demanding than a game, for example...

Anyway, keep it up, Krita is great and can surely take PS's throne easily :-P