Might actually be an issue with my specific install of Windows 10, but nonetheless
Been meaning to play this game for some time, so downloaded it on a whim some time ago and immediately ran into an issue - the moment gameplay starts my FPS locks to ~19-20 despite being unlocked before, no matter the resolution or graphical settings, while still being unlocked in the menus

Happens on every build that I've tried, both Steam version (1.0.314.0, compiled 23rd Feb 2006) and Retail (1.0.320.0, compiled 06th Mar 2006), even in demo

From July 16th to the 22nd, I'll be running the Sharkstream Mid-Aughts Shooter-thon to raise money for my wife Natalie's bills and medical expenses. For each donation of $20 or more, a new game will be added to the queue to be played from beginning to end. If we hit whatever our donation goal at the time is, I will also add Duke Nukem Forever to that queue, also to be played from beginning to end.

Our first stream will be on Sunday, July 16th at around 6:30 PM CST on https://www.twitch.tv/acidxshark

Hope to see you there!!