It actually is much faster, when set to normal priority. My PC is scanned in around 1 hour instead of before 8 hours. But sadly, when set to low, it also takes around 6 hours with 8.8. So yeah, it's faster, on the price of usuability of the PC.
The fullscan depends mostly on the performance of the harddrive. So if you have a slow harddrive (or bad chipset/s-ata drivers), then the users feel the scanning much more. This is one reason why we switched in our new computers to an SSD as boot and application drive, the users don't even feel the scanning anymore.
What you are describing is very unusual. By design 8.8 should yield much nicer than 8.7or any previous version. The most important bit is that you should make sure they scans are set to scan at "Below Normal" which is the ePO default or "Low" priority. You should expect to see 100% CPU utilization but the VSE should yield to anything else running very gracefully.
As a test, try removing Cookies from the scans. I've heard anecdotal reports from one of my customers that cookies has problems. (If you find this to be the case then I'd request you get a MER of the machine and open a case for it.)
Also, it is crucial that any scans you perform of the filesystem are always preceded by Memory for Rootkits and Running Processes. If you don't then the scans of the filesystems are unreliable.
We are having the same issues but I have tracked it down to 2 issues so far.
Older machines are struggling on cpu processing which I have been able to control through EPO and the cpu usage setting. Windows 7 machines are struggling with very high ram usage - IE, the 4gb installed is fully used and then the paging file activity reduces the machine speed to a crawl. Still looking for a fix to that.
A machine wih 4GB RAM being used fully? I would love to see a screenshot of task manager or process monitor with the specific value to that McShield is adding. Other than VSE 8.7 RTW it is almost impossible to make McShield ever climb above 100MB except when it is performing the actual DAT update process. Normal usage for VSE 8.8 would place its working set at roughly 30-50MB. During a full ODS at normal level (which has a maximum number of threads) you can probably make that number climb to about 125MB. Throttling to Low also has the side benefit of less memory usage because there are fewer threads. Those are more what I'd call "normal" or expect values. If you are off those by a large mark then there's something else at work.
I'd love to know a bit more about your configuration so we could figure out what is going on.
I also had a lot of problems with older Computers (older Hardware with just 1 or 2 processors (CPU)). For these computers I set the scan over night with full perfomrnace that it does not stress user during his work. This problem I just have also since updated from 8.7 to 8.8. Before older computers with scan performance 30% was no problem. But if I set it on the old computers also to low, it's just to much for the very old hardware. So I scan these PC's over night with full performance. Thats the way I do at the moment. An newer computers I do not have these Problems.