I have an OK laptop but it chugs once the file is longer than 25,000pixels (I work at 300 DPI in case I ever am fortunate to print my comic...), so I break my files into roughly 3-5 panels a chunk. I storyboard at a low dpi so I can work out a good flow for the panels (like 10 panels), then split them up. Example:
This works for me great but I've had to learn proper file organization, otherwise I'm lost in a sea of files -__-
Once you're done you can load all your high-res files to Croppy:
https://knicknic.github.io/croppy/
which will automatically resize down to Webtoon's 800 x 1280px requirement per page, so no matter what size/page lengths you work at, Croppy will take the hard work of formatting off your hands!
I hope some of this helps! Half the comic battle is finding a workflow that doesn't make you want to rip your hair out... haha!