Lots of room members can cause the browser to crash during autocomplete

[turt2live]

for the devs: typing [at] in this room crashes Riot... as there seem to be a lot of names that the autocomplete tries to show and firefox refuses to render them all and kills the render process for me

https://matrix.to/#/!DgvjtOljKujDBrxyHk:matrix.org/$15216557001832JYKqi:matrix.ffslfl.net

[turt2live]

also affects chrome (as per #6394)

[lukebarnard1]

Previously, we didn't autocomplete after 1 character for users (i.e. "@"). Now we do, it reveals how unperformant it is to render 15k things.