Hi there!

the TL;DR

What system components are most vital to run vMix in UHD mode, handling 2 (or more) 2160p video streams (via a HDMI capure card)?
I assume the graphics card, specifically the amount of VRAM is a major concern?

the long story

I'm running vMix (4K edition) in a mid-sized streaming studio downtown and we usually only do HD productions. It's a small PC featuring a Blackmagic Design 4x HDMI 2.0 input card, an Intel i7 8600K CPU +16 GB RAM and a GTX 1650 Super. So, it's a capable system but certainly nothing high-end.

Recently I used one of our A7sIII cameras for a UHD master shot. Camera HDMI out was set to 2160p, vMix was set to UHD, the external output remained at 1080p and so did the RTMP stream.

(I used that UHD master for cropping in and do virtual zooms with the "merge" transition btw).

GPU mem usage hovered around 68 % I think (the GTX1650 Super has 4GB of VRAM). The CPU didn't crack the thirties. Everything worked well!

Then I switched a second camera to 2160p HD and suddenly I had a craaazy delay in vMix! Audio was still good (very low lag) but the image lagged massively (felt like almost 2 seconds). GPU mem jumped to 90 % (I don't recall the CPU usage but it wasn't crazy high). Everything was still smooth but the lag was of course insane.

I do have an RTX 2080 Ti from my workstation at home laying around and we thought about throwing that into the vMix PC. Would that help? It has 11 GB of VRAM and a higher clock of course. (It's a bit of a hassle to open up and change the internals of our streaming PC for several reasons so I don't want to "just give it a try").

OR could it be something else?
OR is it generally just not a good idea (or "not feasible") to do this?

MANY THANKS!
Oliver