And even if we're talking about 20% of your time, improving that 2-10x is nothing to shake a stick at. 20% of 10 hours is 2 hours, if Vim makes you edit 2x faster, that's 1h saved, per day. And that's without accounting for other work you could move to your suddenly more efficient plaintext mode, and the work you don't even consider doing because your editing experience is too painful (it sometimes work the other way too; there are things I won't do in Emacs, but I will in IntelliJ, where project-wide, language-aware refactors can be done with two clicks in full confidence that nothing will break).
Vim still isn't my exclusive dev editor, but it has almost become my exclusive terminal editor. Learning older technologies may not always be top priority, but it can do a large amount to enrich your personal perspective- particularly if you discover the niche where its usefulness is best utilized (and like you mentioned, if the amount of time to get up to speed will save you proportionate future time).
Anything you're not good at it is going to slow you down. The question is that even tho you might be slow, if you put in the effort, learn it then get good, will the savings and speed up make up for the initial slow down?
I'm a vim fan and won't try to convince others. But I personally believe that the answer is yes. One of the secret to being successful with vi is that of mindset. You need to have a positive mindset that it's worth learning, you need to commit to learning it. I learned vi when network speed was very slow and systems didn't have enough memory. It was faster than any other editor. So I had the mindset that I needed to learn it. if I was starting out today, it probably would be hard when you have editors like vscode.
I have typed and used computers everyday for 25 years non stop and I have no issue with my fingers and I believe a major part of it is using vim and my fingers not having to travel and move out to find a mouse or hitting Ctrl+ keys.