Anyone else notice when tabbing out and back into EQ that is take a while for it to reload the screen. Just stays black for a bit.
It sounds like you updated Windows 10 to 1803. Click your Windows key (or the Windows icon in the task bar) Click the gear icon (Settings) Click System Click Focus Assist Click 'Off' (Edit to add: You can surely utilize Focus Assist by leaving it On if you have a need but will have to modify the necessary settings for it according to your use case. I simply turned mine fully Off because I don't care about it.)
I had a severe case of this problem for a while, and after much putzing around with various settings discovered that my windows desktop resolution was different from my in game resolution settings. after I made them the same it cleared up 99 percent of the problem. I don't know if this will work for you but it might be worth investigation. Good luck.
Any other ideas if this doesn't help? I'm sure this is the latest windows update I was forced to install in Win 10, but I still have a 2-3 second delay when I tab back in. Screen resolutions are the same. Focus assist is now off.