check your CPU usage (lila bar when ctrl+c) when drivin. When the bar gets full your CPU has not enough power or is throtteling down. Have same problem on my Intel Quad 6600 and AMD Q955 i.e. with 79 mod at Brianza 10K with too many AIs.
CPU usage is about half way (Purple Bar) or just over,GPU (Green Bar) is full.And i'm running with default 9 AI. But for me when i turn off either shadows or mirrors the problem goes away.