It’s not the Audio drive as you say , it’s your system drive and that’s the one that counts as all your drivers are installed on that drive . C8 has been rewritten , new architecture so it’s grin and bare it for the version .0 as normal but on most systems it seems to be running very well , obviously it’s not perfect but ive not had any spiking or graphics issues .
I would be tempted to update your Hdd 
From what I have read, 5.9 is the best I can expect from a HDD. Drivers are loaded into memory, and I’ve not experienced any disk related performance problems. I suspect the re-engineered software is the culprit. I have no problems with 7.5 or Sonar X3. When the problems occur in C8P neither my disk, processor or memory are being taxed, yet the Cubase performance meters red line on average load and peak. Since resources monitors show that the system isn’t actually being taxed, it makes sense that this could be in the re-engineered code.
I have two colleagues who use Cubase. We’ve all upgraded to 8, but only one of us is not having problems. It makes logical sense that some will and some won’t, given the nature of the re-engineering Steinberg has done.
What is the problem exactly?
I’m asking because I really don’t know what you are seeing. Is it the ASIO performance meter, does it stutter, are the graphics slow and what is slow? The opening of windows, plugins or meters being sluggish?
If you are on win64x, did you update from Cubase 7.x if you did, did you install it the same way (meaning when the installer asked, install for current user or all). If you mismatch this (user or all) it will use different paths, and Cubase 8 could be using Cubase 7 resources, like skins and dll’s.
I’m not trying to be a dickies 
but I think a lot of the problems could be solved by reinstall using the full installer, and having a updated computer.
It is obvious that many users have issues due to system setup, hardware, third party software or install procedures.
I am one of the lucky ones who rarely has had any issue with Cubase since C4 other than small bugs with the releases.
I just follow directions, pay for my software and everything works.
Weird…
this has decided me to take my puter to my puter shop when it opens next week and have my W7 OS installed clean, I will then install CP8 only…nothing else, this will sort me once and for all…
but what if CP8 is still playing up then 
…ooerr
In my case, the performance meters red line (not the disk one). The sound stutters, pops etc. Loading single VSTs puts the meters between 30 & 50%. No problem in 7.5, only 8. And again, no issues with Sonar X3.
In terms of the graphics upgrade, the re-engineered code requires Aero and is offloading work to the GPU. I switched to my on board GPU as this had better performance with Aero than my Radeon 7450. However, in doing so, the overall CPU usage became elevated. It did show a slight improvement on the performance meters, but no way close what I get with 7.5. In 7.5, I can normally work on quite complex projects at 24/96 without issue. I already did a clean install… total hassle and didn’t change a thing to be fair.