They were reduced to 40v40 because they aren’t open world zones anymore. AV has always been 40v40 just like IOC. The only other epic that was like wintergrasp was original Ashran which was also reduced to make it playable as an epic.
The game can barely handle 40v40 on low. Changing the graphics won’t do anything because any epic bg lag is processing lag, which wouldn’t make a difference if you reduced graphics.