I just updated to Local Beta 5.7.0+local-beta-4616 and clicking on the START SITE button takes about 10-15 seconds to start the site. Opening the second site takes about 5 seconds and opening a 3rd site is about a second.
On the whole this seems much slower than last beta.
I experienced similar performance issues when I updated today, with very high CPU consumption when starting a single site – I didn’t even try with multiple. Running some plugin updates within that site caused further CPU spins, and eventually Local simply crashed and shutdown. I do not have a Pro subscription, yet the Instant Reload option is offered to me. Disabling it for that site, rebooting the machine, and restarting the site in Local seems to have returned performance and behavior to the what I saw with the previous beta.