Reason is most likely because you're running D3D or any of the other hardware selections when you first install UT99.

OpenGL is the (current) choice for most up to date graphics cards to make the game run at 100% normal speed.

This guide here: https://steamcommunity.com/app/13240/discussions/0/846945579778791510/ will most likely fix the problem. All credit to "Turbo Nozomix" on Steam for this.

Also a comparison video I made of the two (D3D vs OpenGL) too show the difference of speed they provide and can make runs invalid.