It looks like the @matrix flagship server is currently under attack by far-right trolls.
All of the larger rooms are currently flooding with pornspam and slurs.
There are known issues with joining large rooms, such as the official matrix.org room.
Largely these are already being addressed by the latest developments such as sliding sync which only load the parts of the room history needed to display the current messages on screen instead of the whole history.
Once you move to a sliding sync capable Synapse with a sliding sync server this should be a thing of the past, although it's just in testing at the moment.
It looks like it was just the couple I tried to join. the matrix chat and the free software chat seem to be targeted right now.