I have an extremely old (ancient) system which I haven't tweaked, fiddled with, or upgraded, for the past 6+ years and it runs FS 2004 smoothly to my satisfaction, locked at 20 FPS.
It slows down a bit now and again in busy areas, but because I am not paranoid about it, I rarely notice the drop in frame rates, and if I do notice it, I take it in my stride and just wait to clear the area.
FS 2004 is wonderful for running on a lowly computer system, and is highly recommended for Folks having "problems?" with FSX...

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I certainly don't spend my time during flight with my eyes glued to the Frame Rate Counter!....

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My Lowly System:
Home build Desk-top Case.
Socket A, AGP, Gigabyte Motherboard.
AMD, Socket A,
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.