So it may be obvious now after playing in portal and as some in this topic have already mentioned - I think it all boils down to having 128 players in one game. I just got done playing a portal match with the newly added BC2 vs BF3 on 2042 maps and the game ran flawlessly 16 vs 16 with my processor staying in the green the entire match with medium settings.
Dont get me wrong - this is not the solution as the game still needs major optimization, but I am having a much better experience now in the smaller modes (infantry only) that I actually prefer. This is why I cant wait for them to add more to portal mode with 2042 specialists on smaller maps that will give XP, ribbons, and mastery unlocks. I think 128 players is overkill especially with the amount of vehicles and how open the maps are.
You can still have an amazing experience with 64 players as even one of the older DICE devs tweeted that they think the sweet spot is between 40-64 players as they had done extensive testing and found 128 and even 256 to not be as fun as they thought