Are you're running Windows 10, and Unreal Tournament GOTY is ridiculously slow, and takes 3 years to load?

Set it to OpenGL instead.

To access in game, Options > Preferences
Usually the first tab is the Video tab, you'll want to be on the Video tab if not. Then click Change by the Video Driver to get to the Video renderer selection screen.

I would generally advise against using the original OpenGL renderer that is included with UT99. Chris Dohnal maintains an enhanced OpenGL renderer, which is significantly better. At the time of writing, the latest version is 3.7. Simply download utglr37.zip from that page, extract OpenGL.drv and replace the existing instance in UT99's System folder. The settings are explained here.

Chris also provides an enhanced Direct3D 9 renderer. In my experience, it works best for old, integrated GPUs (Intel HD2000 or worse). It may also work better with old AMD GPUs, given their poor OpenGL drivers.

All that said, users with modern GPUs are generally better off using Marijn Kentie's Direct3D 10 renderer. Aside from the obvious benefits of using Direct3D 10 as a baseline, it reduces input lag as compared to the other renderers.

Not the point, mine was a lazy OOB solution.

I had issues and used the NGLIDE emulator to clear up the slow load and chugging. Google it quick download and run. Then select nglide as an option in preferences (change render). Looks better and runs great!