Sorry was unable to find the duplicate post..
vxassist move has a requirement for some free space to be available in the diskgroup, this is because vxassist command would be creating some temporary plexes on the free space of diskgroup & use that temporary space to keep date & move it permanently...
Once you run the move command, you should see some temporary plexes being created in "vxprint -qhtg <diskgroup>" command...
So regarding your query, I believe the resultant slowdown will not be worse than mirror.....
Hope this helps
Gaurav