Since you are familiar with RenderEvents, launch RE, select X264 and click on blue gear button as shown.
This brings up the Encoder Settings dialog where you can design your own template. Once happy, enter a name and in the dropdown box and click on the green + button as shown. In this case, I've selected the RGB version of MagicYUV.
Back in KwikProxy, select Custom and then select the template.
@wwaag - O...K... I'm wanting to speed things up within KP, I'm using the latest MagicYUV 2.3.0 as the "feed" to FS and so on, now, I do get the Preview, but once it HAS played out and I hit replay I get another OLD Preview. How do I clear-down OLD Previews from another Project?
Regarding MagicYUV 2.3.0. That version is NOT being used. As I stated before, you CANNOT use a native Vegas template. MagicYUV is being encoded inside of FFmpeg which is different from the version that works inside of Vegas.
To get rid of old previews, open the Preview File Folder (click on the folder icon in the KP dialog) and delete all files named preview.mp4 or preview.avi.
The built-in template using MPEG-2 (XD CAM) at a bit rate of 25Mbps which is probably the fastest CPU only format. Since you have an Nvidia card, you could create and try an Nvenc template. In RenderEvents, open the Encoder dialog and select h264-nvenc as the Encoder. One thing I just discovered. Do NOT use the "speed" preset--it will lead to a crash. Nvidia recently changed the preset name from "speed" to "performance". My suggestion is to just leave it set to default. I am about to release a new build, so I'm glad I discovered this bug which unfortunately applies to most rendering tools.
You could test to see if the Nvenc encoding is any faster than the built-in MPEG-2. The processing bottleneck will most likely be Vegas since it does all of the Fx processing in order to create each "frame" which is then sent to the FrameServer.