I went ahead and updated my 10.13.4 partition and tried ESO. I then saw a cursor problem and fps ranged from 24 to 35 with pauses of half a second here and there. It was not really playable. However all my software runs worse on 10.13 (the worst MacOS I have ever used), so I dont consider it an ESO problem.
Performance on my machine are higher than before, in dungeon I stay at 60fps capped with all max playing in 4K. IN open world it vary from 30 (in complex fights) to 60fps. So I can say the game is performing ok, can't be better considering it runs on OpenGL.