If you are experiencing stuttering since Summerset, you may need to disable add-ons which could be affecting performance.
- Go to the "Elder Scrolls Online" folder in My Documents.
- Open the "live" folder (NA megaserver) or "liveeu" (EU megaserver).
- Remove all files from the "AddOns" folder.
- Remove all files from the "SavedVariables" folder.
- Remove AddOnSettings.txt from the "live" folder.
You can find some additional troubleshooting suggestion in this thread.