This may be a duh obvious question, but where might I start with trying to identify why two nearly identical computers have rather different performance data.  
 
Computer 1
https://www.dropbox.com/s/8tznjjdvmk2nf6y/perf_na4.png?dl=0
 
Computer 2
https://www.dropbox.com/s/27hbo70u2s2d4dc/newaud3_perf.png?dl=0
 
Both have Windows 7, 64 GB DDR2 memory, SATA main disk, SSD sample disks.  Both have the same processor and motherboard.  (i7-5820 and Asus x99pro)   They differ in the audio and the video.
 
The project is the same in both images, no live audio connections involved with either -- it's a midi project using Kontakt sample libraries.
 
#1 has a USB audio interface, TASCAM USB 122mkII, which seems to work fine and silently.
#2 has a RME audio card, PCI express RME Hammerfall AIO, which though very powerful has a really annoying ground hum all the time
 
#1 has a NVidia GTX 750 card driving 3 monitors
#2 has a more powerful NVidia card  GEForce GTX 780 TI driving 4 monitors, 3 of the the same as #1 plus another.
 
The first computer has visibly more trouble with the project (as shown in the performance image) and it hits audio dropouts from time to time; the second computer sails throught the project without ever a problem. 
 
What makes the bigger difference?  Sound card?  Video card?  Something else?  
 
I'm getting set to take them apart and trade around parts, but that's a process fraught with danger of upsetting the routine.
 

As I post this, I do notice some memory differences, even though they have loaded the same project and have the same total memory.   Not sure if that might be the video card difference or what.

The answer to your question is the audio interface. A good audio card with good drivers makes a world of difference in performance. RME makes some of the best interfaces in the world, with the best drivers and it shows. Additionally when you go from USB to PCI(e) that can lower the load as well.
 
The GPUs are likely not at factor at all.

I wish I could get rid of that RME hum somehow; makes using headphones impossible.   The only solution I've found so far is running a second computer to take the SPDIF digital signal (which doesn't have the hum, of course) and transfer it to analog and on to my amp and speakers.
 

How do you know it's definitely the RME that's causing the hum?
 
Have you swapped out ALL cables for known good ones?
 
Are both "identical" computers connected to the mains in the same way?

I've done a lot to try to identify the hum, but you're right it could still be something I haven't thought of.   I've tried different power adapters, different house circuits, different cables.   The RME customer support was dismissive, blaming the motherboard manufacturer (ASUS).   No way to know whether they were right or wrong.

williamcopper
This may be a duh obvious question, but where might I start with trying to identify why two nearly identical computers have rather different performance data...
What makes the bigger difference?...Something else?...

 
It could possibly be the software rather than the hardware. These factors may or may not be relevant, but I would compare the following lists:

• Installed programs (in Control Panel)
• Processes running (in Task Manager)
• Background Services (in Computer Management)

Have you checked power options in BIOS and Windows? Check with something like CPUZ frequencies of CPU/RAM/etc when playing the project.
Also check disks performance, I have seen buggy SSDs which while working was extremely slow.
 

The differance is just what Sycraft said, The audio drivers are important. Why do you think RME is the top performer in the interface world. 
 
The hum
Have you tried a USB cable with a Ferrite Choke?