So far the consensus is that mastodon.social is very slow because of the influx of users, but the jury is out as to whether it will speed up again? Do I have that right? Trying to figure out if I should plan on migrating.

If I understand it correctly, when you migrate to another instance, you can take your followers and follows with you but not your existing posts, and you also have to change your messages on all your other sites and networks about where to find you on Mastodon.