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A
blue clock? This sounds like the "show turtle" - feature, although I'm not sure, cause I've never seen it by myself. It's not the "lag" clock for bad ping, this should be white.
What I would recommend to you first: There is a file
config.cfg in your Nexuiz\data folder.
Please open it with a text-editor and add this line:
showfps "1"
Save and close it and start Nexuiz.
From now you are shown in the bottom right corner, how many frames per second you get within the game. This is not for solution, but will help for further diagnistics.
Next step - please, for this and every other forum where you want to get support, always post your hardware specs (at least CPU, RAM, video adapter, OS). So when you post how many fps you get in the game, don't forget to write us about your hardware.