We are aware of the fact that many computers can't handle M2 trees on the strat-map, mainly because the RTW engine is somewhat antiquated. The stance of the team on this is that they are a considerable improvement to the visual experience, but if you would prefer to bring back vanilla RTW trees (which are quite ugly, by the way) in order to lessen the lag, here is a solution for you:

1. Download the file attached to this post

2. Place it in (Program Files/The Creative Assembly/Rome - Total War)/RTRVII/data, replacing the previous file (you may want to rename the previous file, in case you prefer the M2 look).

3. Load up your campaign and enjoy!