I'm so glad you asked. first of all, here's the wikipedia definition for Hor+

"Hor+ (horizontal plus) is the most common scaling method for the majority of modern video games. In games with hor+ scaling the vertical FOV is fixed, while the horizontal FOV is expandable depending on the aspect ratio of the rendering resolution; a wider aspect ratio results in a larger FOV.[8][9] Since the majority of screens used for gaming nowadays are widescreen,[10] this scaling method is usually preferred as wider aspect ratios do not suffer from reduced FOV with it."

and for vert-:
"Vert- (vertical minus) is a scaling method used by some games that support a wide variety of resolutions. In vert- games, as the aspect ratio widens, the vertical component of the field of view is reduced to compensate. This avoids distortion of objects in the game world but results in a smaller field of view on widescreen resolutions, and may become especially problematic with extremely wide resolutions, such as those common on multiple-display setups.[citation needed][13]"

so what does this mean?

it means if you want a wider fov, or if you play triple wide or in 21:9 aspect ratios, or anything other than 16:9, you'll need to apply this fix.

The same goes for literally every udk game out there.

To do this you'll need to find your game files in:

Your documents\my games\(game title)\??game\config\???engine.ini (and systemsettings.ini if it has it)

For killing floor 2 it's:
C:\Users\?????\Documents\my games\KillingFloor2\KFGame\Config

hey!
ever want to know how to change a unreal engine 3 game to be hor+ versus vert-?
just find this in the ???engine.ini from:
[Engine.LocalPlayer]
AspectRatioAxisConstraint=AspectRatio_MaintainXFOV
to
[Engine.LocalPlayer]
AspectRatioAxisConstraint=AspectRatio_MaintainYFOV --CHANGES TO HOR+

Now we'll force all of the textures into memory and uncap the streaming.
change these in ???engine.ini:
bUseBackgroundLevelStreaming=False *may cause problems with machines that don't have enough vram
MipFadeInSpeed0=0
MipFadeOutSpeed0=0
MipFadeInSpeed1=0
MipFadeOutSpeed1=0 *these change the mipmap fade in
PoolSize=999999999 *THE WORLD FAMOUS TWEAK! even if usebackgroundstreaming is set to true (for video cards without enough vram) this will force the entire level to your cache, disk, memory, you name it! It usually improves performance.
AllowScreenDoorFade=False *stops things from being removed when they are behind you
bAllowSwitchingStreamingSystem=True
UseTextureFileCache=FALSE
**IF YOUR GAME HAS A ???SETTINGS.INI SWITCH TO THAT NOW
OnlyStreamInTextures=True

After your tweaks, save, and right-click the ini's that you edited. Click on properties, and set to read only. This is essential, especially if you turned off background level streaming!

WHAT THIS DOES:

These tweaks force all of the textures into memory from the get-go. This reduces latancy and frame lag by a huge extent, but frames per second is usually the same.

THIS WILL DO LITERALLY NOTHING FOR YOUR FPS
But fps isn't everything...
BUT CHECK OUT MY IMAGES HERE:
http://i.imgur.com/Mg3a3z8.png

this was taken from a fraps benchmark in the first level of Rise of the Triad 2013 in Fraps, and made into a chart in FRAFS, before I fixed the streaming system

http://i.imgur.com/SjuV6Ej.png

and this is it after. notice how flat it looks by comparison, while the fps didn't change? That's the difference in frame lag.