Hello
First thing - using 4.3 is not a must. 5.5 converts 2k3 sources, too (even w/o R2)
Second - 4.3 does not encrypt the data traffic by default (see Increasing the cloning performance for details). SSL encryption is not the only performance factor; check disk I/O (both on source machine and destination ESX), network throughput, other processes using the resources, etc...
Regards
Plamen