GNOME 40's Shell Theme Code Is Rather Expensive But Optimization Pursued
In particular, Daniel noted that he noticed with GNOME 40 that up to half of the render time is being consumed by the GNOME Shell's theme code. He is now investigating possibly rewriting the slow parts of the theme code as a shader in order to cut down that render time. This work though is still in the early stages with no merge request yet or any guarantee of success.
With Ubuntu 21.04 having stuck with GNOME 3.38 rather than using GNOME 40, there has been less exciting upstream optimizations to report on by Daniel / Ubuntu but for Ubuntu 21.10 that will presumably tick back up. It's good to see them now evaluating GNOME 40 performance shortcomings. Plus with Ubuntu 22.04 LTS now being less than one year out, we'll likely see more attention paid to ensuring GNOME's great performance for that release.
He has also been working on other GNOME Shell optimizations, wrote a new GNOME Shell theme background optimization, started on a stencil code redesign to make that code faster, and other improvements.
More details about some of these latest GNOME optimizations being pursued can be found via the Ubuntu Discourse.