Complitely agree with Martin and Stephen - check your PC, and not only browser.
It is very common than enterprise customers complain about slow response of vSZ GUI, even so servers running VM are usually very fast.
It looks that UI uses client PC heavily, so responsiveness is mainly dependent on administrator PC performance, not on server. If you have a slow PC, are low in memory or just have 30 another browser windows opened, it may become really slow.
Not getting response, you click on the same menu again, and get "no response" error. It is frustrating, I agree, but usually can be easy fixed.
I have seen it many times in enterprise environment, where every PC is bloated with different corporate applications (from antivirus to background compliance checkers, messangers and updaters), which slow down PC quit a bit.
So when UI is slow -- look on your PC resource utilization statistics, and you'll probably see reasons. Some housekeeping may greatly improve performance -- close some applications to have more free RAM, delete temporary files in Temp directory, so on. If doesn't help - remove plugins and reset Firefox and use 64-bit version, not 32-bit.
If you are still short of resources, use different PC or add RAM and replace system drive by SSD. Cheap upgrade (< 100 Eur) -- 120 GB SSD as a system drive + 8GB additional RAM usually makes wanders. Nowadays SSDs are so cheap, that using standard HDD as a system disk has no excuses - it means that you just intentionally waste your precious time...
For me an my colleagues vSZ works well with both Chrome and Waterfox, without any issues and reasonably fast, and it worked well through all vSZ/ vSCG versions from 3.0.3, with different browser versions during this period, even when I am connected over Internet to remote vSZ.
Hope it helps.