Welcome to TVPaint Community forums !

Whenever I preload a sequence at either 1080p or maybe 2x that resolution, preloading is very very slow.
(Something like 15 minutes on a fast system, lots of ram to load a 1.5 minute long video. Importing a video at it's native resolution to a new project with preload ON)

Any way to improve the speed when pre-loading for the next builds?

I don't think it will change as we use ffmpeg technology.

but hint : it depends on the codec of the video. so maybe it's an explanation (because I don't have such a slow import here)

Can you recommend a Windows program to convert my clients movies to FFMPG on Windows 10 x64? Handbrake?

Handbrake does a good job, indeed.

TVPaint only works with uncompressed data (unless you DON'T preload, but that has it's own quirks)
Importing 90s of 1080p @ 30fps will use :
90*30 = 2700 frames * 8Mb/frame = 21Gb of RAM...
Flushing this to the tmp file is what slows you down the most.
Is the tmp file on a really fast HDD/SSD, and is the file you're importing on ANOTHER disk ?
(And I mean disk, not another partition of the same disk).

Could you recommend any codec or setting which would be ideal to import it in TVPaint?

Image sequences

Thank you Elodie!
I assume if possible JPEG sequences, cause they are much smaller and faster to read then PNGs?

Depends on what you'll do with the imported frames.
- JPEG is lossy (even at 100% quality !), good for reference material that won't show up in the final product, because it's smaller than PNGs.
- PNG in non-lossy, better for material that WILL show up in the final product.