Hi
Converter can throttle network traffic, jobs can be configured to do so. However the default is no throttling, I assume you haven't set it explicitly.
Parallel tasks are rather meant for disk I/O optimization, not so much for network.
Unfortunately I don't know what to suggest to optimize cloning (except excluding SSL encryption which you already have), I think you should focus on ESXi network performance tuning and/or disk IO tuning.
E.g. this section from https://www.vmware.com/content/dam/digitalmarketing/vmware/en/pdf/techpaper/performance/Perf_Best_Pr... is relevant since Converter also uses NFC:
To migrate powered-off virtual machines and, in some cases, to migrate “cold” data (the base disk, and any snapshots other than the current one, in a virtual machine that has a snapshot), Cross-host Storage vMotion will use the Network File Copy (NFC) service. As in the case for powered-on virtual machines, the NFC service will similarly preferentially use VAAI or the source host’s storage interface. If neither of these approaches are possible, the NFC service will use the network designated for NFC traffic.
NOTE In-place upgrades of VMFS datastores with non-1MB block sizes to VMFS 5.x leave the block size unchanged. Thus in this situation it would be necessary to create a new VMFS 5.x datastore to obtain the performance benefits of 1MB block size. NOTE Prior to vSphere 6.0, NFC traffic could use only the management network (sometimes called the “provisioning network”). Starting with vSphere 6.0, NFC traffic still uses the management network by default, but can optionally be routed over a network dedicated to NFC traffic. This allows NFC traffic to be separated from management traffic, giving more flexibility in network provisioning. In any case, we recommend that NFC traffic be provided at least 1Gb/s of network bandwidth.
HTH,
Plamen