Outside of Big Picture Mode, the Steam client start-up performance is improved in general, improved CPU performance at runtime for Steam Input, and many bug fixes. The Steam client beta for Linux has improved download speeds on high latency connections, the SteamOS compatibility dialogs will now show in the desktop UI when installing games not verified for that hardware, and the desktop Linux UI will also show the SteamOS unsupported dialog when installing games that are known to not work with Proton. The Steam beta also improves its Linux distribution information collection for the Steam Survey to be more accurate. The SteamRT3 experimental beta has also seen several fixes.

The ridiculously slow start-up time has been an annoyance for ages, so I just switched to the beta to see if it's noticeably improved.

I did a quick unscientific test, timing start-up first on the standard client and then switching to beta, restarting my PC and timing the start-up again.

The non-beta time from double-clicking the icon until the UI was fully rendered was 25 seconds, and with the beta 18 seconds. Fingers crossed that's enough to make the start-up delay less bothersome in real-world usage.