We're basically doing nothing but fixing Synapse's performance problems currently, and improvements are on the horizon. We're late because we got sidetracked fixing flaws in the protocol, but as of the last few weeks we're back on perf again. So far we've sped up /sync by 2x and reduced CPU by about 20%, but the RAM improvements should land over the coming weeks unless we have further distractions.
Meanwhile, on Riot, we're busy landing performance fixes which should make it much less resource hungry than Slack. These are literally landing as we speak: https://github.com/matrix-org/synapse/pull/2970, https://github.com/matrix-org/synapse/pull/3331 etc.