I experienced very worrying problems when I synced/installed some Typekit fonts, luckily resolved now. For the record I have a new-ish 2016 iMac which has been running very smoothly and fast, 4 GHz Intel i7 processor with 32Gb RAM.
I synced/installed several fonts from Typekit, but got an error saying conflicting fonts, saying it was trying to add some font versions which I already had, which I thought I had resolved. After that my computer would not start up as per normal, it was taking 5 minutes or so to start when before it was almost instant, it was freezing a lot, the desktop was flashing black, dock was disappearing. Tried restarting but it was taking ages.
I looked in Activity Monitor and noticed 'fontd' was taking up a lot of CPU, which was never as high before. Not entirely sure what that is but looked into it and its clearly related to fonts, so this looked like my problem. I rebooted in Safe Mode and this has appeared to solve the issue.
Why would adding Typekit fonts cause so much hassle? Surely it should be easier, and even if there were conflicting issues should it just not instal them and resolve itself (like Suitcase used to do), and not cause such serious issues? I'm not keen to use any Typekit fonts now.