Curious if you tried with extensions disabled? I just attempted opening both a 100k and 1m line html file in both VS Code and ST, in the 100k case they both were quick to open but Code was a bit smoother in scrolling, in the 1m case ST took a long time to open (showed a loading progress bar for a few seconds before anything), and while Code was choppy to scroll ST was almost unusable.

That being said, with extensions enabled it can be a totally different story - some extensions will try to load the text content on every editor opening or even edit, which gets unusable very quickly. (Yes they run in a separate process, but that’s not a cure-all)

Tested on 2018 MBP.

(On VS code team, had nothing to do with the editor perf)

Are you referring to the way VS Code is slow to display the syntax highlighting on file open sometimes? (Is that when it's getting highlighting via LSP?)