While I'm not exactly sure what may be causing this issue, I have a couple of hints which might work:
1. Go to your usersettings.txt file, located in your Program Files\Elder Scrolls Online\Live
1.1 - Open the file and locate SET PreferExclusiveFullscreen "0". Set it to "1", if it's not already.
1.2 - Locate SET FULLSCREEN_v5 "0". Set this one to "2".
2. If you have windows 10, make sure you have fullscreen optimizations OFF. Helped me a lot on performance aswell as the time it took for me to alt-tab. You can apply this in general (check a guide on the internet), or you can apply to a specific application. It would help running the game through the .exe and not from the launcher:
2.1 Right click ESOx64 > Properties > Compatibility > down below select last 2 checkboxes. Also, under the last checkbox, which should be "Run this program as admin", you should have another option you can click mentioning "PPP settings override" (May not be the same, my W10's language is set to Portuguese). Anyways, click it and check the last checkbox.