So over the past month and a half of testing we've seen quite a few different tests and most seemed fairly ineffective at helping with the lag. However, one test in particular, week 5's test which limited support and healing skills to only your group provided a noticeable improvement. While there was still lag at times, in general it was far better than what we've seen over the years.

There are definitely a few downsides to how this test played out though. First, it made players who wanted to run around and try to support their faction without joining a group basically useless. Many of my healer friends expressed their frustration during that week that they may as well not go into Cyrodiil without a group otherwise they wouldn't be able to accomplish anything. Second, it put the organized groups that already run 12 person raids at a huge advantage compared to normal which could have a negative effect on the more casual players that are joining zone groups as those zone groups will have a far harder time than normal. Third, you are still encouraged to run the largest size group possible, and while that's only 12 they should be trying to encourage smaller groups that may have a lesser impact on the server than the 24 man zergs.

My suggestion is to raise the group cap from 12 to 20 or 24. This will allow zone pick up groups to still remain fairly effective and have a numbers advantage against the organized 12 mans. That said there should be some mechanic besides prox det to deter them from wanting to stack that large of a group. One mechanic I had thought up was essentially a mechanic where the smaller the group is the more ultimate they can get when fighting outnumbered. Perhaps some mechanic where in a group of 6 or less they receive 1 ultimate for every enemy they hit, 12 or less they receive .5 ultimate per enemy hit and then above that they do not receive any extra ultimate per hit. This will allow a smaller, more organized group a better chance against the large groups but will not reduce the effectiveness of large groups in general. The last suggestion is to make groups under 4 people able to heal and support players outside of their group, this will remove the situation where the solo healer feels helpless when they enter Cyrodiil but prevent large groups who tend to cast more heals and things like that on their allies from creating too many calculations on players in the area.

While this may not give quite the same performance boost as we saw in week 5 of the Cyrodiil testing it will still hopefully give a decent amount of performance improvement while also shifting the focus from running the largest possible group to running smaller groups that can potentially be more effective. ZOS will still need to make many back end changes to help further make large scale fights more performant but this could be a good first step.

TL;DR:
Raise group cap to 20 or 24 from 12
Make some kind of balance of power between small groups and large groups to help promote running smaller vs running max size groups
Allow groups of 4 or less to heal friendlies outside of group.