In general yes the Linux version is slower than the windows version - largely I think do some of the libraries we use are slower than their windows counterparts. (Although over time some of these libraries are improving so some of this performance penalty should be reducing)
However It hard to tell if this is the slowness you are seeing. If there is a specific part of PT on Linux that is slow then its possible we can optimize it. (although its unlikely to be done in 7.6 due to its status).
Are you using 14.04/trusty or 16.04/xenial?
There are general things one can do to get better performance out of a VM - but given in general if your happy with your VM performance then these suggestions are unlikely to help.
