If you windows virtual machine is slow to boot, chances are you are running the incorrect HAL (Hardware Abstraction Layer).
Most commonly, using a multi-CPU hal while only running a single vCPU in the Virtual Machine.

The rule of thumb in the virtual world is use virtual SMP with caution.

If the virtual machine does not require two vCPU's then do not give it two vCPU's and if it does require two (or you think it does) start with one first as your baseline.
You might actually find when you give it the second that performance is worse than when it had one.

It all depends on the application and in some cases it is better to scale out than to scale up, you also have to take a look into how VMware does vCPU scheduling if you are considering running many vm's with more than one vCPU. Also ask yourself if the application actually supports SMP ?

The reasons are not as obvious as you may think and VMware has a good KB article on it at the link below :

Slow performance of virtual machines that use vSMP when running on an ESX Server using certain hardware