Why does the virtual machine slow down?

Question

virtual box

Greetings, comrades! There is a laptop Intel Core i5, 8 GB of memory. Why can there be such a problem - there is Windows 7 basic optimized. Memory consumption 700 MB. The virtual machine is 7 maximum, and linux ubuntu 16. Both machines slow down, although 6 GB of memory is allocated. Applications are dull, although the guest OS consumes no more than 30% of memory. Maybe a jamb with the settings, where to look? Should not be so stupid in this mode. On a laptop without a virtual machine, heavy applications such as autocad and a browser work fine. There is an assumption - the hard drive will die soon. Or some settings are not made.