It's not related to defrag. The lenth of time it takes to boot seems related to the number and/or size of the snapshot(s).
When you ran defrag the snapshot grew so your next reboot took a long time.
I was told there is supposed to be a KB coming out within the next week or two but it's not going to tell us anything we don't know. From what I was told, the fix for 6.7 is going to be out later than 6.5. Hopefully vmware will release it sooner though.
For VDI users this might be fixed via an update to some component of Horizon View and you may not have to wait for an updated ESXi version. I don't use VDI so don't know any more than that.