Did you try updating your drivers here:
http://www.nvidia.com/Download/index.aspx
Tick "Clean Install" under Advanced.
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Borderless vs Fullscreen both lag the same now? It used to fix this issue but now doesn't or was just a short placebo?
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Are you playing Multiplayer game modes or Co-Op game modes? Co-Op modes are hosted by users and on poor connections it can cause bad gaming conditions.
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Rename (or delete) t he Documents\PVZ:GW2 folder will reset in-game settings to defaults on next launch. Maybe something got changed somehow?
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Do you have any user.cfg file the base directory of the game (you would have had to put one there as it isn't stock). Maybe something bogus is in it, try renaming (or deleting) it.
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I know you said you didn't have overlays.. but maybe something is sneaky around. Try disabling Origin-in-Game for GW2 as a test. If this fixes the problems something is likely causing issues for Origin-in-Game (needed for features like store, friend joining etc).
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Other programs in the background hogging resources? CtlAltDel should suffice for checking around. One guy I remember a while ago had a coin-miner type virus program causing extremely poor game performance.
