I found three posts mentioning this exact same issue that go back to April 2014 so this is a long standing game issue. I had even logged out of TeamSpeak3 prior to testing this so there were no other programs running on my computer at the time, only ESO. My FPS rates were 45-60 with a consistent latency of 115. Again the exact same issue, one out of 6 key presses would actually cast the spell.

Then I waited for my magicka to refresh and started spamming again, nothing happened again with 0 Addons and everything cleaned out. At first there didn't seem to be a problem. I went through all of the User acceptance screens and loaded the game back up. I then quit my game and restarted opening even a fresh Launcher screen. I also deleted all of the files in my SavedVariables folder other than the ZOS file in there as they were all Addon files. I deleted the AddOnSettings, oked & UserSettings files. This was with Addons so I moved all of my Addons out of my C drive to my destop. At times, I would get one cast out of 6 key presses. While in Rawlkha by the Stable master, I was spamming Breath of Life over and over. I figured I would test the Addons I had to see if that was the problem and it turns out that they have no impact on this issue. You can't fight when you can't cast damage or healing spells.

It got so bad that there wasn't any point staying in Cyrodil. It took one player almost 5 full minutes to be able to Teleport and Mount their horse as nothing was happening when pressing their keys. Multiple people on our TS were saying nothing was happening when they pressed the keys. This was on the PC NA server last night at 8:30 PM MST. Our latency was just fine, 100-160 ish and normally have no issues then. Hello in Cyrodil last night there were a bunch of us trying to battle with the Blues and Reds and our spells wouldn't cast.

Daggerfall how to cast spells