"Slow" in what sense? Development? Because I self host a Conduit server and I don't ever notice messages being slow. It would be hard to notice anyway, as in a group chat people usually take some time to type in their responses.

The sync between large groups used to be slow because of amount of data, but Element X and "sliding windows" were rolled out to help with it.

AFAIK, the public Matrix server used to be slow because of a heavy load (I think), but on my self-hosted instance that's not a problem at all.

The experience of using Matrix involves a lot of sluggishness at various points in the client - waiting to decrypt messages or properly sync keys, waiting to join a room or for room search to load - these are the things that have been salient to me using multiple matrix clients with a freshly-spun-up server within the past month.