The VM is optimised for the OS, the OS is usually a fresh install with just that 1 program you need to use instead of you're entire life scattered across the desktop, it can be a snapshot of the system in an optimal state right after running an unfuck windows script that removes default system malware which doesn't let it reinstall, it has less system resources to deal with for the simple fact it can't use them all at the same time as the base OS.