Hello Alex,
> And yes, we really have 5.1 :-)
I was more referring to the build number of 5.1. Is it U1 already? At least some performance improvements were implemented in the later updates. Unfortunately Update 1 also introduced a couple of fixes to SSO which actually allows users to log in now when this was not possible before with the caveat that the general login time might actually take longer depending on the identity sources and ad group membership of the user.
> everything seems slow - login, opening the menu tree etc.
But as you seem to experience the performance issue even after log in this should not be the main reason. Are you using Chrome on the the external clients as well or a different browser (IIRC Chrome was not in the supported browsers list but should be working without too many issues anyway).
Did you try different browser? IE 8, 9, 10, FF, Chrome for example. Do you see a difference? Was the Web Client the only tab opened in the browser with the only instance of that browser running (just to rule out general browser performance degradation).
Is the performance better when you login with a user that is only an SSO administrator (admin@system-domain without vCenter permissions for example)?