Hi Vida -
For the moment, I wouldn't touch the number of Tableau processes that are in play.
In your shoes, the first thing I would check is to see what the IO looks like on this VM. It is not uncommon for a VM's storage to be on some sort of shared device (like a shared SAN or a "big bunch of disks") which multiple VMs access. if one VM "hogs" this resource, you suffer.
If "your" VM can't get fast enough disk response of enough throughput, The VM, and therefore Tableau will be slow.
Run any free disk benchmarking tool on your VM, then do the same thing on a physical drive (maybe on your laptop or something).
If the VM's benchmark comes back "low", I'd investigate that first.