What Can I Do with Seeds?
There won’t be any “major speed issues”, and if you’re writing good JS code, generally you shouldn’t find it much slower than C or C++.
Most of your overhead will occur in the database/file system of a typical application. If you plan to do complex calculations, you may find discrepancies.
In short: it depends on what you do with it, but overall, I wouldn’t say you’ll see major speed issues. It works well for me.
If you think it can be done in Python, then JS can too.