Well, I might as well ask my tech support question here :)

I just ran the upgrade to 6.0.0, and it downloaded so many things concurrently that it killed my wifi (old router). Is there a way to cap bandwidth or maximum concurrent connections? (this is something I have to do in many download heavy apps, e.g., steam)

I don't think homebrew allows throttling bandwidth but it does let you set maximum concurrent downloads though. I believe the default is twice number of cores; you must have quite a few :) Set this to your preference:

HOMEBREW_DOWNLOAD_CONCURRENCY