LoL.
In JO you can have 4-5 helicopters flying around shooting, armoured vehicles, boats, people spraying away on emplaced weapon's and up to 120 people in a game.
I would be very surprised if you did not get low frame rates every now and then.
It is not just your vid card,CPU and ram which affect performance.
ISP connection, Virtual Memory settings, hard drive performance and the number of programs running in the background will also affect performance.
The performance might also be affected by FRAPS. I had an old version which slowed JO down. Needed v2 to get it ruining OK.