Roblox takes about 12-14 seconds to open a game from the site after one of the recent weekly updates. I believe it was 629, but may have been 628. There’s also a very obnoxious white screen during this time. This has persisted through Roblox reinstalls and PC restarts.
Before then, games took about 3-5 seconds to launch.
I have tracked this down to being an issue when SteamVR is installed. it’s worth noting my Valve Index has all its ports plugged in, but it’s unplugged at the break-off cable.
Removing or disabling the SteamVR runtime fixes this on my machine. I’m currently doing this using GitHub - zachup100/SteamVR-Manager. Hope this helps.