1. You probably have a specific problem with drivers that's causing your tablet operations to be very inefficient. I would look into getting new Wacom drivers to start with. Focus first on why your powerful system is running poorly with the tablet. Determine whether a mouse works better than the tablet, to ensure you've isolated where the problem is.
Then, after working that out, if performance is still troublesome to you...
2. I would suggest that, assuming you have the budget, you relegate ALL your spinning hard drives to be backup or low access storage drives. Do NOTHING with them for normal, everyday use. See item 3 below for that.
3. Get 2 TB of SSD storage in the form of four 512 GB SSDs, put them in a RAID 0 array - not RAID 5. Make that drive C: and install Windows, all apps, your working data, scratch, swap, etc. Don't worry about reliability - get good ones, pro models. They have high MTBF, and even with 4 SSDs you will not be taking nearly as big a chance of failure as with a single HDD. Re-evaluate whether the RAID controller you're using is a good match for high throughput SSD storage and get a good one if not.
4. Institute a thorough backup regimen. That sounds a bit scary, but it really boils down to setting your system up to do system regular system image backups to your HDD storage (or an offline USB drive). RAID 5 wastes resources in an ongoing way in anticipation of failure. Backup prepares you for any eventuality, and RAID 0 uses the resources efficiently - especially SSDs.
5. You have 32 GB of RAM, which is sufficient, but make sure you've done whatever is needed to ensure your system maximizes its efficiency in accessing that RAM. I'm not familiar with that ASUS motherboard, so I don't know what it can do, but sometimes you can dig and find info on optimum configurations where when the right things match, the system switches into a higher mode with more channels, etc. I know my workstation does special things when the RAM all matches.
I've personally done all the above, save for the tablet diagnosis as I don't have a tablet. There is NOTHING that will wake up a system - every aspect - like having a high throughput I/O subsystem. You'll wonder how you ever could stand using HDDs.
Thanks for your reply! Your help is really appreciated.
I've done the testing with another tablet and with a mouse. That did not resolve the issue - still laggy.
For now I will start heading forward to putting all system rapid access data (TEMP, Scratch, swap and so on) into a single dedicated SSD (Think it will be 256Gb Samsung 840 PRO), cause all SATA ports except one are occupied for now.
Then I think I will transfer the Barracudas into a dedicated Q-NAP NAS storage and start building a SSD subsystem for everyday work with a data backup to that NAS.
Thanks again for confirming that a "bottleneck" is a HDD IO subsystem.