Chalk me up as yet another happy user of Pinboard.
I put up with the ridiculously slow Delicious bookmarking for far too long only migrating to Pinboard two years ago.
I have about 18k bookmarks on pinboard and speed has not suffered at all. Bookmarking is very fast.
About the only criticism would be that search of my bookmark tags seems a bit more primitive than on Delicious.
Can I actually search multiple tags (not text) in my bookmarks? For example all bookmarks with hn and programming?