The forum may need to know a little bit more.
My experience has been that any i7 should be sufficient for most situations, but you might let us know what processor and how much ram.

My first guess would probably be Your graphics card. vMix is very dependent on the GPU. What graphic card are you using?

Finally, what capture device are you using and what is your capture rate?

This forum can help a great deal, but we'll need a little more information.

If you use the fullscreen output from the videocard, you get the 2-3 frame latency.
When you use a sdi output card, the latency will be higher, as the system needs to copy each frame from the videocard to the sdi output card.
Bear in mind that the 2-3 frame latency is from the input to the output on vMix only. Everything else in the chain will add up to the total latency.
So try to eliminate other switchers between the vMix system and the projectors. Some consumer and prosumer camera's can have latency's of multiple frames.
When using dome camera's, hanging them upside down with image flip on, will add latency to their outputs.
Projectors will also add latency, which might even be higher when using image enhancement. Always look if they have "game mode" or "low latency" in their image settings.
Geometry or other advanced functions within the projectors can also add latency, so put the projectors at the perfect position and try to avoid using any image enhancement engines.
Tell clients there will be latency when they do their request. That way they won't freak out at the moment itself while they feel under pressure, thinking there is something wrong.
If it is a big venue, the audio won't be in sync everywhere anyway, because light travels much faster than audio. 50M or 163ft will already give you 145ms audio delay, so lots of times it isn't such a big problem anyways.