riot hangs when you open a large room

[richvdh]

for example, #_rizon_#horriblesubs:matrix.org. Possibly because there are lots of membership events, which then get collapsed, so we end up paging in millions of events

[bwindels]

Good to have a channel to try lazy loading on!

[dbkr]

Actually I think this is the same as #7163 (I initially thought it wasn't)