The lightweightness here referred to the complexity of the fork - i.e. it's not an attempt to entirely rewrite Riot, but instead a reskinning and UX simplification exercise. The intention is to be able to keep merging in updates from Riot proper (and indeed to port stuff back into Riot).

Agreed that Riot itself uses way too much RAM though - but we have some massive improvements on the horizon there; by lazyloading user data on demand rather than preloading it up front, we can improve RAM usage by ~5x. This work is happening over the next month or two (modulo GDPR).

Meanwhile, you can always use a desktop client; Nheko, Fractal and Quaternion are all looking increasingly good :)