We've got a lot of these nowadays in our environment. I've started splitting out the very large VMs (4+TB) to their own subclient, and that helps a lot. There's also an additional setting that I've set on my media agents to allow multiple streams on each VM. Name: "nAllowMultiStreamsPerVM" Category: VirtualServer Type: Integer Value: 1. You'll never get more than one stream per vmdk, but this will let the vmdks run concurrently.
The biggest help that I've had is with changing the backup type to hotadd vs NBD. Not sure how you've got your environment set up, but I'd highly recommend using SAN or HotAdd transport if you aren't already.
Best of luck!