Fascinating! I started another vMotion on another VM and waited for exactly half an hour from the start time (not a second off!) and vMotion moved forward from 14%. So sequence of events:
1. vMotion starts and it reached 14% quickly.
2. It stays at 14% for exactly half an hour
3. It moves forward quickly from 14% to 98%
4. It stops at 98% for another half an hour
5. vMotion completes.
I searched values in Advanced Settings but did not find anything that's set o 1,800 seconds (ie half an hour)...