This hasn't been an issue up until now. Usually when using Krita, the fill tool would fill the designated area instantly with no issues. However, now when I use the fill tool, it takes about 5 seconds or more to fill, no matter the size of the area. A loading bar shows up at the bottom right with the words "Flood Fill: %". Why is this happening all of a sudden?

I really like Krita and have gotten used to it, and I don't want to stop using it. But these persistent issues are really driving me away from it.

My specs are:
Intel Core i5-9300H (CPU)
NVIDIA GTX 1650 (GPU)
16GB RAM
1TB HDD
250GB SSD

Well, if it's a new issue, then it's not a persistent one yet I implemented some new stuff in the fill tool. There are two features, make sure to set them correctly:
Sample: either Current Layer or All Layers (the new one is called "Color Labeled Layers" and it could be potentially a bit slower)
"use selection as border" (I think it shouldn't be available in Krita 4.3.0 yet) - disable it to get the same performance as before.

If it's not that, please check the older version of Krita and the new one on the same picture to see if it's really much slower on the new version. You can use portable versions for that.