I am able to run the R46 at around 15-20 FPS on my Mac Pro desktop (ATI Radeon 4870 graphics card).
I usually get around 35-45 FPS on most openBVE routes/trains.
On most other games, I get a solid 65-75 FPS.
It may be a problem with openBVE's renderer...
Not too updated with desktop cards, but I'm sure a 4870 is dated.
Though, back to my laptop; the 525M, I've been running consistent good FPS in CS:GO, BF3 and such. OpenBVE has only maybe dropped to an average of 50-60 FPS while in cab mode. The only train that gives me an even more drop in FPS is probably the R46, though it's only a bare 5-10 FPS drop.