Having adequate capacity on your HDD is important, but that won't change the speed of the drive. When RAM fills up, the computer creates an overflow file on the hard drive (the page file) to store the excess data. If this file isn't on a fast drive, it will slow down everything. Instead of moving all your data files to a different drive, the simpler option might be to install a small SSD and tell Windows to move the page file to it. But I would try maxing out the amount of RAM your system can support first, as that will avoid the entire page file problem to begin with.