Hi everyone!
For those who have not very powerful PC and using Windows 10 or, even maybe 11, and who might experience lags on later Windows OS, I found a solution for you of, how to deal with lags. At least this solution worked for me, so I hope that it will work for you.
So, if you experience any lag in original Red Faction Guerrilla on Windows 10, 11 or later, then try to do this:
1. Go to your game's folder and open it up.
2. Right click on rfg_launcher.exe and go to "properties".
3. In "Target" field, after the path of the game, press Spacebar once and then type this command as it is: -nod3d10
It should look something like this: "F:\Games\Red Faction Guerrilla\rfg_launcher.exe" -nod3d10
4. Then apply the changes and launch the game via that same rfg_launcher.exe and try to play it.
That should deal with lag and game should work smoothly now. But, if next time you trying to play it and it lags again, then do this:
1. Exit the game and wait 2 seconds.
2. Launch the game again and quickly skip every intro video, without letting it even play.
3. Start the game or load your save file.
And there you go. This should help with your lag problem. At least it worked for me. But I have to always relaunch the game, every time I'm returning to it, to make that -nod3d10 command to work. I don't know why it happens, but it happens. Anyway, I hope it will help you too, if you experienced lag in OG version of the game.