after about a month of happy using nexus dock it suddenly became very very very slow. even menus are fading in about 5-10 seconds for a menu. dock sometimes not responsive at all. my system is an i7 with 64GB of ram with 4GB quadro card display.... what happened? what can be done?

Open Preferences,General tab. What is the Animation Speed slider set to?

Also, if you open the Windows Task Manager and go to the Details tab, what percentage of the CPU is Nexus using (and for this to make sense I also need to know how many cores your CPU has - quad-core, right? - and if Hyper-threading is enabled or not).

Finally, have you already tried ***temporarily*** disabling your Anti-Virus software to see if that helps? What did you change - or update - in your system recently? Did you reboot your PC already?

performance is set to maximum as this system has enough resources
I also tried upping the priority to above normal (didn't help).
of course I've tried rebooting, re-installing, and there's an exeption for the AV list.
I run a dual monitor setup, only one dock on left display. not even set to auto hide.

also I've noticed that usually when the dock becomes slow, when mouse is over the dock and it's not responding or very slow, the taskbar reappears (it's set to "hidden" in the dock's preferences).

Ah, that might be an important clue.

Windows has a 'nasty' habit of making the taskbar visible again on a lot of events, so when the taskbar is set to hidden the Winstep application has to constantly monitor it to make sure it doesn't become visible and hide it again if so.

Perhaps the Winstep application and the taskbar are constantly fighting with each other, which might explain the slowness? I have never seen this happen, but, just in case, next time the dock becomes slow try disabling rhe 'Hide Windows taskbar' setting to see what happens. There is even an internal command you can add to the dock that does this, if you want.