I wanted to piggyback off of this forum posting, because I am basically seeing the same problem on a new rig I started building this week. My first Instant Replay rig actually... I started using vMix in 2013 and love it to death, but never seemed to have a wish to use the Instant Replay functions until now...
Similarly to chrisis, I really only start to notice a problem when recording and playing back simultaneously in a 2-camera instant replay scenario. My best guess right now is that my storage is somehow being a bottleneck, although I have a very fast SSD dedicated to Instant Replay storage. I also managed to find a reproducable Replay UI interface bug that I will share later in this post.
My hardware is as follows:
Asus Z10PA-D8 server motherboard
2x Xeon E5-2620v4 quad core @ 2.10GHz
Intel SSD 750 Series SSDPEDMW400G4R5 400GB PCI-Express 3.0 NVM disk (for replay only)
NVIDIA GTX 750 Ti
Cameras being recorded are 2 NDI network sources inside my LAN
Project & Replay settings are 1920x1080p 59.94
vMix 18.104.22.168 x64
My total CPU usage is ~30% when running vMix with the replay recording & playing. Render times hover between 4-12ms. In the statistics window, the "Source Dropped" climbs about 5 frames per second. I have relay channel A & B set to camera 1, and if I switch channel B to camera 2, the stuttering/dropped frames gets worse. I've already run the vMix Diagnostics tool and it found no issues with running programs.
I was considering buying a GTX 1080 card, but given the results by chrisis, I feel like I want to hold off for now since my render times aren't high and thus don't feel that would help this problem.
I'm definitely hoping to find a solution to the dropped frames when playing out, I feel like this disk is plenty fast.
Additionally, a separate issue mentioned above, I found some funky behavior when adding the 2 NDI cameras to the replay settings screen. If I add them and specify 25Mbps recording, the 2 replay inputs are created successfully, but when I hit record I get this error...
And the instant replay controller shows no input information...
If I go back to the Replay Settings and change it to 100Mbps (for example), then the input information appears and the recording begins properly. I can even change it back to 25Mbps at this point and no recording error is seen.
Thanks to those who have replied to my earlier threads, and thanks Martin for continuing to update and support a great product.