It's ALWAYS smarter to add old runtimes as new additional runtime instead of copying old junk into fresh install.
And it you have a runtime so big it takes forever to load, you really need to start using multiple runtimes, and break up those big monsters into manageable runtimes.

I have all my installed stuf, and all my work runtimes fo rall my products, and I have no speed issues at all. I have about 30 or so runtimes, and just swap between them as I need to.

Way too many people take way too many things way too seriously.