After doing that "Driver Boost" thingy (which found several very old USB drivers, among
other things), updating my ATI Crimson Driver, and changing some settings there, and in
"Workshop", I have now been able to fly a campaign air combat.

The frps varied between 25 and 45 frps mostly, only looking at clear sky gave me 60 frps.
It must be the ground, that has become more frames-hungry - when I look to the ground,
the frps shrink dramatically.
Despite the micro stutters, I was able to fight, and to win my fight.
Still, there is a lot of room for improvement...

I'll carry on trying things out and will report.