I've been putting some time in doing standard scenarios on the Horseshoe Curve route. A few things I've found.
1) With most routes, in most free roam scenarios, I get a pretty rock solid 22-30 fps (Using TSX, high graphics).
2) However....the framerate reported does not change much at all with changes in graphics settings (using TSX). It really makes no difference whatsoever what my graphics settings are at, this includes all the graphics settings in game (including shadows and bloom, DOF), as well as resolution. I can set the resolution to 800x600 and put all the graphics settings to the low presets, and I get an average of 25 fps in the first standard scenario. I then can save it part way through, exit the game, put the resolution to native (1680x945) and set all the graphics to the high preset, and I still get an average of about 22 fps. A little less, but certainly not what one would expect.

EDIT: Later discovered I had "incorrectly" set up my Nvidia graphics program settings.......it was limiting my framerate.
3) The second scenario (fire) brought the game down to an average of 7 fps. I saved, exited, unchecked TSX, and resumed the scenario, and it was steady between 38 and 42 fps. So, I definitely agree theres really something odd about this behavior that should be looked at
very hard by RSC.
4) On the third scenario, throughout the entire scenario, my framerate would be at 25 fps or so for about a minute, then go to about 15 fps for a minute. It would do this throughout the entire scenario. I monitored my CPU and GPU, and I saw no changes in temperature or usage when it would drop or increase framerates. It didnt seem to matter what was in draw view (I thought maybe another train passing would cause a dip in fps, but not so).
I'm starting to see what everyone is talking about. In free roams, TSX seems to behave pretty well. In scenarios, wow...
I too will be using non-TSX until the fix TSX. Its just too frustrating to put up with...and some of the things it does and doesnt do seems to defy logic on a computer that is fully capable of running Microsoft FSX, ARMA II, and other similar games at high graphics settings...
I honestly believe that if they can (and do) fix TSX soon, they will truly have an unbeatable simulator. First class in graphics by far, and lots of room for improvement to make it first class in physics, too. But if not, I cant imagine this will be good for business in a couple of months...TSX is their future...whos going to buy an addon for 30 bucks when they know ahead of time it will be frustrating to use?
At the very least, maybe they should ramp up their minimum requirements to an i5, GTX360, and 8gbs of ram. I would consider my performance in scenarios to be 'minimum'.

I sure hope they get past this bump in the road!