extremely slow encoding

In addition to what Neil stated, it may be that Premiere Pro 25.1 needs something that the GTX 1080 Ti doesn't have (hardware-support-wise) in order to export at a sufficiently fast speed. And with Nvidia pretty much leaving support for pre-Turing GPUs hung out to dry, it would not be surprising at all that a forthcoming version of Premiere Pro would require hardware Ai support in discrete GPUs just to even render or encode with hardware acceleration; in which instance the 1080 Ti would soon be permanently locked to the software-only rendering and encoding modes.

That seven year old, 8 core CPU doesn't have any hardware bits for long-GOP/H.264/5 encoding and decoding. The 1080Ti does have a goodly amount of vRAM, but the other tech is a bit old also.

So for ProRes, it's probably ok, but any long-GOP isn't going to be fast.