Unexpected low harddisk rates are usually caused by the os' File Caching mechanism. The OS detects which files are often used and puts them into free ram. You probably have lots of free ram with that 32GB, if you aren't running memory intensive applications next to resolume.
You'll see disk io go up if you make resolume use files that havn't been used for a while (trigger different clips), or just use very long videos that dont fit into free ram
The low cpu / gpu usage you're seeing has two causes. First these programs tend to show you an average over a certain amount of time. Resolume pretty much uses each hardware resource sequentially. So when we're using the cpu, the gpu will be idle. This idle time shows up in your measurement too.
The second cause is that they show an average for the entire system. Your cpu is built for multitasking, it can do 8 things at the same time. Resolume doesn't have 8 things it can do in parallel all the time, so a few of your cores/hyperthreads are just snoozing while the others are running as fast as they can. (you probably wont see this in your tool as the os switches threads between cores to distribute heat, so it'll show as averaged out between multiple cores) This comes down to resolume 5 preferring 4 fast cores over 8 slow cores, yours arent slow so you should be okay there.
In the end it's always hard to pinpoint directly where performance issues are coming from and it's always a combination between software and hardware.
If i have one gues concerning your hardware, it would be memory bandwidth. The video frames dont travel from gpu directly to the capture card. Instead they travel from gpu all the way to ram, and then from ram all the way to the capture card. It's actually pretty hard to saturate a PCI3x16 bus, but it's a lot easier to saturate the ram io. Since you said you have no disk io, i expect the os to use file caching, adding to the burden of the ram usage.
I havn't found a tool yet which supports proper memory profiling, so it's hard to actually verify if this is your bottleneck. If you find a good tool please let us know