First I've heard of company-mode, thanks for that. A DDG search gives https://company-mode.github.io/ as its first result, on their it refers you to its issue tracker - perhaps let them know if you haven't already?

Large CSVs, Perhaps use LFV (large file viewer) if you want a read-only look.

Otherwise I just loaded a 26MB binary file in less than a second. I created a 2.46 Gigabyte (not MB) CSV, opened it in a new emacs, took less that 15 seconds (fundamental mode). I think your problem lies elsewhere - it may be an unnecessary mode being invoked.

How did the browser run when you turned off JS, a whole lot faster I imagine?