Hello,
We will need more information to answer you the questins. The boot time of VM depends on alot of things. Maybe the guest operating systems has some resource heavy services which make it boot slower, maybe some of your VMs are on slower storages and that is why they boot slower, maybe you have insufficient resources and after you power on some virtual machines there is not enogh resources for the rest, maybe you have some hosts with slower CPUs on the cluster...
