Thanks for replies - patch 7 downloading as I type.

And in return: I too get huge slowdown whenever the battlefield is raining. Around 1 frame a second (or less) but not so bad that I can't hit 'Wait' and get the next day's weather coming up in a bearable amount of time. Been the same with all patches of Rome II, and the same when running under both 'Ultra' and the next down settings.

CPU: i7-3770
GPU: Radeon 7950 (3 Gb)
Mem: 16 Gb DDR3 2400 MHz