Incremental replication throughput is slow when target proxy uses Virtual Appliance transport mode (HotAdd).
This issue is caused by VMware's limitation. Writing to VM snapshot in hotadd mode may cause high I/O load on the target datastore and result in latency spikes and slow throughput. Network transport mode is usually much faster in that case.
Re-configure the target proxy to use Network (NBD) transport mode.
Veeam v8 is not affected by this VMware bug, v8 no longer uses VMware VDDK API function VixDiskLib_Write() for hotadd mode.
Veeam is currently addressing this problem with VMWare.