Symptoms:

Opening file slow.
Hiding/showing layers slow.
Drawing, even on new blank layer slow.

Image Specs:

<10 layers
Uses around 400Mb RAM
8bit color space
Uses glow layer style

Troubleshooting attempts:

D3D, OpenGl, and no hardware acceleration (no discernible change)
Reduced from 4k to 2k (some speedup, still unusable)
Converted to 16-bit (no discernible change)

Image File: https://1drv.ms/u/s!ArrmcYKakZcQhMcq15W_wfaevaSI2w
Video: https://1drv.ms/v/s!ArrmcYKakZcQhMcuT3dZSA_s0tN_vw

Computer Specs:

Ryzen 5 1600
GTX 1600
16GB RAM
OS, Krita, and Paging file on SSD but rarely used.

Krita

Version: 4.1.7

Qt

Version (compiled): 5.9.3
Version (loaded): 5.9.3

OS Information

Build ABI: x86_64-little_endian-llp64
Build CPU: x86_64
CPU: x86_64
Kernel Type: winnt
Kernel Version: 10.0.17134
Pretty Productname: Windows 10 (10.0)
Product Type: windows
Product Version: 10

OpenGL Info

**OpenGL not initialized**

Well, apart from the layer style -- layer styles do have a performance impact because they need to be calculated whenever a pixel changes, nothing weird stands out.

There is nothing but what's in the FAQ that we could tell you, and apparently you've already tried that. So, obviously, something is wrong with your system: maybe you're running sandboxie, or an electron app like spotify has taken up all available memory, or something else is hogging memory or cpu, or whatever.

Or you're just unlucky and have a system where Krita cannot perform well: Tyson Tan did a survey last year and tested a dozen cheap Windows systems, and on some, out of the box, everything was fine, and some were built in a way that Krita didn't perform well.

But that's not something we, as developers, can do anything about at all.