X-Plane 11.0 public beta 8 went up over the weekend. If you are seeing problems with the flight model, please report bugs ASAP so Austin can look at them! I think we are approaching the end of some major flight model updates; the last thing Austin is looking at is better body shadowing, which he will probably write up shortly.
Over the weekend, with the help of some very patient users, we found the cause of poor performance on some AMD hardware*. I have a prototype fix, and I hope to have it in X-Plane some time this week. This fix will only affect users who were seeing super-lousy performance on very light configurations.
In the most recent betas, the threaded driver no longer totally kills X-Plane performance. But it does still slow things down a little bit on some computers - I see 5% fps loss with the threaded driver on. My suggestion for now is: try it both ways and run with whatever works best for your machine; this bug is affected by both your particular CPU and the kind of work-load your settings induce in X-Plane.
I am still working on improving cloud performance, and a recut of the DSF tiles is rendering as I type. The first priority for the new DSF tiles is to fix blatantly broken tiles (e.g. YSSY) and get the file size down so we can put Alaska and friends back.
* I wouldn't call the AMD problem a bug on either our part or AMD's - it's more in the category of "OpenGL makes no promises as to what might be fast, so app developers have to debug on all shipping hardware and try not to do painful things.