They’ll still be slow, as I really doubt that anyone will manage to reclock the cards, ever. Hopefully, now that they’re supported by the NVK driver, which is likely getting a lot more attention than Nouveau’s OpenGL driver and supports Vullan, performance might improve.

According to Collabora (main developers of NVK, I think) & a few other sources (I haven’t run any benchmarks myself), NVK + Zink (OpenGL on Vulkan) is a bit faster than Nouveau on newer cards, for which it is now the default. The older cards still default to Nouveau, but as I understand it, it’s just in case there are issues with them.

So, they are a bit nore compatible now. They should also be a bit more performant now (if you use Vulkan instead of OpenGL & NVK works) or soon(er or later) (when NVK gets faster or the cards switch to Zink on NVK).

Nvidia drivers are a bit confusing on linux, so I’ll list them here, just in case (at leas as far as I understand them):
Kernel drivers:
Closed source official driver
Open source official driver
Nouveau Nova (WIP)

Userspace drivers:
Official driver (supports both officiql kernel drivers)
Nouveau (OpenGL only, supports the Nouveau kernel driver)
NVK (Vulkan only, OpenGL via Zink, supports the Nouveau kernel driver, will support Nova)

Kernel drivers run in the kernel & talk with the GPU, while userspace drivers talk with the kernel drivers, as far as I understand.

Both NVK & Nouveau’s userspace driver are part of Mesa.

All the Nouveau drivers are often just called “Nouveau”, but they’re all located in different places. (Nouveau also has a Xorg driver, which isn’t important here.) Also, the package for NVK is (at least on some distributions) called vulkan-nouveau (Arch) or similiar.

TL;DR: The situation should be at least a bit better now and it should be easier to improve. Also, Nvidia drivers on Linux are confusing.

P.S.
I have a device with an intel integrated GPU & one with a Nvidia 2000 series GPU. Everything works with the intel one (and I would assume AMD), but Nvidia sometimes causes problems on the other device.

It was some time since I tried NVK on it, but performance was much better with the official drivers. It’s probably much better now, and I’ll probably have time & motivation to test it in a week or few, after getting a few other things out of the way first.

Depending on your card, luck, setup, needs & distribution, Nvidia could just be a minor annoyance (enabling non-free repositories and/or some manual configuration with the official drivers, or not quite enough performance with Nouveau) to larger problems (broken greeter/DE/WM/etc., problems with secure boot signing or something else with the official driver or horrible performance or lack of support for extensions with Nouveau).

If you’re looking for a GPU, I’d recommend avoiding Nvidia. Official drivers work pretty well now (most of the time), but can cause headaches and the amount of time I’ve spent troubleshooting is not completely insignificant. There are still a few (pretty small) problems I haven’t been able to resolve. This is in contrast to the intel iGPU, with which I don’t remember any problems.

Edit/P.P.S.
Sorry for the wall of text.