We get around the shared folder performance problem using NFS. Adds one extra step to setup (mount a network share), but we've never had performance issues with it.
Not so familiar with vagrant on mac (though my time is coming), but having used loopback KVM's on rhel I can say that fiddling with mount options can drastically improve stability / performance (though still much slower)
e.g. tcp mounts, getting read/write blocks matched up btw/client server and sized to be digestable but big enough to move data, etc.
also, nfs is mainly only suited for 'NAS-like' operations - things like rdbms's do waay better on iscsi or eating the vdisk performance.
last I messed with macos nfsd (which has been a while), it a way happier with smaller blocksizes (e.g. 8-64k range) - modern linuces will attempt 1MB which is too much for the older 4.4BSD based code
another thing to look at is timeouts / backoffs - it's easy to kill performance by setting these things too agressively so that the system double-chokes when it gets bogged down..
I used NFS when using vagrant shared folders a lot a couple of years ago. Didn't really pay much attention to the performance, I must admit, which was fine for my purposes; for me it only really needed to crash less than the default shared folder stuff...
I did have to supply an extra "mount_options:['actimeo']=1" option to the synced_folder command to make file watcher-type systems work tolerably. With the defaults I'd often be waiting several seconds before any changes made by the host would be picked up by the guest.
