Couple of symptoms here:
i) Running ESXi 6.0
ii) Running with the maximum amount of RAM and vCPU count
This was an interesting one insofar it would only occur if if you had updated the VM’s to use above.
VM’s would upwards to an hour to boot.
Upgrade to ESXi 6.0 patch 1 (build 2809209)
In terms of actual detail never got the time to truly ascertain why the slowness occurred for me – occurred during the splash screen phase.