There are literally multiple big screenshots on the page I linked and a big join button that brings you to https://join.movim.eu where you can chose an instance.
There must be something wrong with your browser. There are multiple big screenshots in a slider on https://movim.eu
And the unrestricted instances on the join page can be used with any xmpp account, however I don't recommend it as servers running Prosody currently only support a subset of Movim's features.
Matrix is slow on large instances, but that's not the case here, especially if no federation is done.
And the issue with sluggishness is currently the main development focus with ElementX/matrixX that will become mainstream matrix soon. With that even the large instances are extremely fast.
Because there are very very few Sliding Synch (which is the part of X that makes it faster) instances at the moment and only one that has a major userbase.....