Table copying is exactly one of the things I've used libraries to provide. While there's a great Wiki article describing how to do it, it isn't the sort of thing I'd want less experienced developers building, even if it is just copy and paste. Mistakes happen even then! I'd rather do it once myself, and do it right. And at least if I don't do it right I have only one place to correct it!
I haven't had a need so far to copy just part of a table, but that sounds like something I'd want to add at some point. I have added some functions for building UI5 tables from JavaScript arrays, though, since that is something we're looking at doing a lot (until Personas has native table support, anyway:-)
As for performance, you are right that loading lots of extra code is going to impact performance. I don't have a feel for how much as I don't have any large libraries. I haven't really noticed an extra load time. But if it becomes a problem, you just need to divide your libraries a little so that if you aren't writing scripts that manipulate tables, you don't load the table library.