The HDD seems to be the main bottleneck. An additional factor is, how much virtual RAM and cores you have dedicated to your VMs. It is possible (and likely with seemingly old HW), that the VM eats up the ressources of your host.
Try to lower the settings of new VMs (RAM and cores). I use VB for fast testing of reported bugs, recreating user issues, testing other distributions, and don’t see such issues. But i use it on hosts with SSDs instead of old HDDs and i give max 2 cores and 8 GB RAM per VM. This is enough.
On a really old laptop, which i do no longer use, i was only able to run two VMs in parallel or it got really slow due to the limited HW of the Host (2 cor Intel with 8GB RAM). So HW of the host limits what you can do with VMs.