That's true, but not really relevant until you hit 1000s of chat members. MLS targets up to 50,000. But there isn't a plausible threat model for a secure chat that large (someone will leak or it will be infiltrated).
At 1000 members a DCGKA key update (when the group membership changes) is ~300kb. So, equivalent to someone sending a small image, which they probably do very frequently.
well, encryption needs computational resources; you have to expect higher overhead when you do encryption in the web compared to native code e.g. in Rust or C. It kind of is the upper bound of slowness ;)
