I bought Fallout and Fallout 2 on Steam years ago, but I've only recently started playing them (the joys of backlog syndrome). But I noticed that both have horrendous mouse lag in full-screen mode and I spent a while fiddling around before discovering this was all thanks to Windows 10's infamous 'Full-Screen Optimisations' that can and has fucked up many a game over the years. Ultimately you can either disable this for both games (though for Fallout 2 it's a bit more involved), or just run the games in a full-screen window instead which arguably is the better option.
To do the latter you just need to enable the option manually in the
f1_res.ini (for Fallout) or
f2_res.ini (for Fallout 2). There are copies of these files in the game's installation directory but the ones that actually get used and need to be edited are hidden in your user account's AppData folder:
In here is a folder called
Fallout2 (or both if like me you have both games of course). In them is another folder with an apparently random assortment of letters and numbers as its name and then inside that folder is the ini file. You can edit it with Notepad and all you merely want to do is change the
WINDOWED_FULLSCREEN option to 1. Make Sure
WINDOWED is set to 1 too for good measure.
Alternatively to this, if you really want to run the games in proper full-screen mode for some reason, you can disable full-screen optimisations instead, which is easy enough to do for Fallout, but as I previously mentioned for Fallout 2 it isn't as simple. Both Fallout games on Steam come pre-installed with their respective high resolution patches, so with Fallout (which is the UK version and censored as a result, though you can fix that by installing this mod) a quick and easy fix is to simply right-click on
falloutwHR.exe in the game's installation folder, select the
Compatibility tab and put a tick in the box next to
Disable full-screen optimisations. The game will now run properly full-screen without mouse lag (or at least it did for me when I tried this method).
Fallout 2 on the other hand also includes the sfall mod which doesn't play nice with compatibility options (supposedly). As a result when starting the game having turned off full-screen optimisations in the same way as Fallout, you'll be greeted with an error telling you the game appears to be running in compatibility mode and that you need to disable that to continue. The sfall mod did include an option to skip these compatibility checks, by editing
ddraw.ini in the installation folder, however it seems the option has been removed and the actual feature itself doesn't seem to work in whatever version of sfall is being used by Steam's implementation of the game.
I did get it to work though, by replacing the
ddraw.dll with one from a newer version of the sfall mod and adding the
[Debugging] section to the bottom of the existing
ddraw.ini and then adding another two lines;
SkipCompatModeCheck=1, which stopped it complaining about compatibility modes being enabled, therefore allowing you to launch the game with full-screen optimisations disabled. I'm not sure if the author of the mod made it prior to the existence of full-screen optimisations (most likely), hence why it porbably doesn't take them into consideration with this check, but it didn't seem to be an issue to have them turned off with sfall enabled when I briefly tested it.
Obviously this all applies to the Steam versions of the games but I expect if you got the games elsewhere or even have an ancient retail disc version and they suffer the same issue, you can still do what I've suggested here. Just remember the full-screen window method only works if the full resolution patch for either game is installed.