I'm running Windows 8.1 (upgraded from Windows 7 SP1) in VirtualBox on Ubuntu 14.04. This setup has been satisfactory for many months. In recent weeks the Windows boot has the hard disk running continually for about 5 minutes. This is far longer than usual. Even activities such as web browsing from Firefox in Windows in VirtualBox seems to require a lot of disk activity. When VirtualBox is running, Ubuntu is now far slower than usual. Anytime VirtualBox and Windows are up there seems to be a greater chance of actions in Ubuntu taking longer than usual or of those actions involving an unusually large amount of disk activity.

Note that Ubuntu is still performing well when VirtualBox+Windows are not up.

I had the virtual Windows drive defragmented on a regular basis so that cannot be the problem. Ubuntu is on ext4 so I don't defragment the physical drive because ext4 is not supposed to need much defragmenting, if ever.

The physical drive is 1 TB. Ubuntu uses about 480GB of which about 430GB is the virtual drive for Windows and 50GB is Ubuntu programs and data. In the virtual drive of 430GB, Windows thinks the size as seen from the inside as being 400GB (rather than the 430GB as seen from the outside) of which 50GB is used for Windows programs and data. I don't know why Windows thinks the virtual drive is smaller than what VirtualBox gave it but don't know of any reason to be concerned. As you can see my programs and data across the two systems totals only 100GB. I can't type fast so a 1 TB drive will last me until I want to throw it away. I think the original saying is "I can't type at 300 megahertz".

I setup VirtualBox with 5GB of the 8GB physical RAM. I allow VirtualBox to use 4 processors of the 4 physical (8 hyperthreaded). I have a 90% execution cap set in VirtualBox. These settings have not changed in a long time so I don't think they account for the deterioration in performance of either Windows or Ubuntu.

McAfee's antivirus scan in Windows revealed nothing.

VirtualBox is updated about 2 times a month. Ubuntu is updated about 4 times a month. Windows was on automatic downloading and updating but since things got slow I turned that off.

Why is my system slow now? The hard drive is now 38 months old so I will replace it with an SSD within a few days. The problem might not exist when I have fresh installs on the SSD. If some adverse condition is present it might be hidden by the sheer speed of the SSD. I wanted to post this question in case there's something that can be fixed on the hard disk setup or if there's something adverse to be avoided in my future setup for the SSD.