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.

@Alex @matrix

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.

@Blort @matrix it all stabilized, but it did seem especially taxed joining the channels under attack. Though, those were large channels, so it might not have been related to the attack.