Booting ESXi from USB performance

Hello,

I've read a few times that once booted most of ESXi is loaded into memory so the performance of the boot media is not so important once it's up and running. Would this be fair to say?

Has anyone booted ESXi from both USB 2 and USB 3 devices and noticed a difference in performance? Is it desirable now to boot ESXi from a USB 3 device?

Thanks.