X-Plane 815 much jerkier than 806

In each case, they're running with all the default settings (including
defult aircraft, default airport) except resolution set to "Extreme" -
815 is unusable. In Spot View it's particularly bad, with the screen
updating barely once a second. 806 on the other hand is perfectly
smooth.

I'm downloading 81 to see if it's any improvement, but in the meantime
is there any easy way to fix the performance of 815?

I'm using a 2GHz iMac G5 with 1.5GB RAM.

Daniele