I use REX for my FS 2004, and it seems to have very little noticeable effect on my frame rates (locked at 20 FPS), even with my lowly computer system...

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I find it useful for loading in real weather for the area I fly, and is surprisingly realistic according to the local weather forecasts in the area!
Nice water and ground effects, and the clouds are wonderfully realistic!
Paul....G-BPLF....FS 2004...and REX, GE Pro, and VOZ... 8-)...!
...I just use GE Pro to lighten/brighten up the scenery below (RGB settings)...

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Win 8.1 64-bit. DX11. Advent Tower. Intel i7-3770 3.9 GHz 8-core. 8 GB System RAM. AMD Radeon HD 7700 1GB RAM. DVD ROM. 2 Terra Byte SATA Hard Drive. Philips 17" LCD Monitor. Saitek Cyborg X Fly-5 Joystick. ...and a Briggs and Stratton Petrol Lawn Mower.