I was trying to add color to a drawing I did and as I worked Krita became incredibly slow as I progressed.
My picture was up to 3.2 GiB in memory when finished and it took it a while to finish applying a stroke as I worked near the end.

My setup has 8 GB of ram and an Intel I7 processor so I am a little bothered that it was becoming so slow.
As it stand I am unsure what to do maybe I need more ram or a graphics card or what but I can't work when the brush is taking a minute to finish each stroke.
I do know that when flattened the picture became 98.0 MiB but when I'm working I do prefer to keep colors on separate layers so I can adjust them
Especially since this was a small picture 7x12in

I wanted to start coloring pictures I have done on 9x12 inch paper but this makes me think other wise.

Any advice?

System stats
8gm ram
Intel I7 processor
Intel HD graphics 630
Windows 10

File details
7x10 canvas
600 dpi
multiple layers
Total file size 3.2GiB
98 Mib when merged

I've just created a large (6000 x 4200 = 10" x 7" at 600 dpi) image with many layers (of dense painted scribbles) that occupied 3.8 GB of RAM and I had noticable but not very bad brush delay (with a fairly complex/large watercolour brush) when I turned Instant Preview to Off and disabled Canvas Graphics Acceleration. I had 4GB of RAM allocated to krita in this situation. I did have a serious brush delay when I reduced the memory allocated to 3GB and I assume this was due to krita going swappy.

My computer is an old dual Xeon 'workstation' but I limited krita to using two cores.

I may be wrong but this suggests that having a graphics card would not help. (It's complicated and a krita developer would be able to offer better advice.)

I think krita sets itself to a 50% allocation of RAM as a default setting so you might try increasing that to 75% (6GB for you) which should be ok given that Win 10 seems to need about 1GB and assuming you have no other significant applications running. This may or may not help you.

Also, would you be satisfied with a 300 or 400 dpi resolution because that would have a noticable effect on your image size.

I totally concur with ahabgreybeard specially on the dpi, if you are creating your illustrations/paintings from scratch on Krita, for the intended printing size and even higher 300 is more than enough, (usually this are settings that has more impact on photography and scanning while "acquiring" and image).

If you also are working on anything above 8bit for the color space, that can slow things down considerably too, (specially combined with 600dpi), again, if you planning on printing, 8bit would suffice, more depth in color is meant for digital display.

One extra tip is that if you use any of those layers with any filter/effect/mask, etc. try to disable them as you paint then activate to see the results, when any of these are active, every time you make a stroke, Krita has to re-calculate the whole effect making it painfully slow. May not be your case but is good to keep it in mind.

I am working with scanned in line-art I drew by hand, the dpi starts out a 600 dpi, but when I actually post it's 300, I save the large scale for archival reasons and for future print use.

I am worrying that this will require me to have to flatten my layers after I finish a section of an image, I basically work by using a separate layer for each color, and then have a separate layer for each major tone, leading to a minimum of 7 layers a color, more if I add a separate shine and shadow color.

The reason I keep them separated this way is both to protect the previous pass and so that I can change the color if necessary.

I've tried adjusting the settings in Krita to no avail, and I am unsure where the Preview option is.

Any other advice?

Instant Preview is a checkbox under the main View menu. You can try it to see if it makes a useful difference for you.

I can understand your desire to work and archive at a higher resolution and I do that myself but at a much smaller scale than your artwork. I suggest that you make a copy of your .kra file and then try scaling it down to 400 dpi (as a compromise resolution) and then see if working at that lower resolution helps at all. It's the sheer size of your image (and its many layers) that is causing problems.

If you've tried increasing the RAM allocation to 6GB and that didn't help then there seems to be no point in spending money to give your computer more RAM.

Apart from that, all I can say is Good Luck.