NFS issues since upgrading to 13-RELEASE

I posted this in -questions and someone suggested I post here as well.

I'm having NFS availability issues between my Proxmox client and FreeBSD server (10G link) since upgrading to 13-RELEASE. And unfortunately I upgraded my ZFS pool to v2.0.0 before I noticed the issue, so I'm kind of stuck.

Periodically, the NFS server (I've tried both v3 and v4.2 clients) will go unresponsive for several minutes. I never had this problem on 12.2, and as far as I can tell it's not a disk or network I/O issue. I'll get several "nfs: server not responding, still trying" messages on the client and a few minutes later it usually recovers. It's not clear to me yet what's causing the block. Restarting nfsd on the server will resolve the issue if it doesn't clear itself.

Any pointers for troubleshooting this? I've been looking through vmstat, gstat, top, etc. when the problem occurs, but I haven't been able to pinpoint the issue. I can get pcap, but it would be from the hosts, because I don't have a 10G tap or managed switch.

-- 
Chris