[CentOS] Centos 7 as gateway - UDP performance is busted/awful?

I just replaced a dead system disk on my KVM host that was
running an ancient fedora 13. Since centos 7 was available,
I decided to go with it to get some long term stability.

The problem is that NFS mounts inside the virtual machines
don't work for spit when talking to older NFS servers that
must speak UDP.

Is there something about UDP traffic that requires tweaks
I don't know about for centos 7 to serve as a gateway machine?
I've got the ip forwarding settings and other sysctl stuff
that was set in the old fedora 13 system.

I've got the bridges defined that same way as the old f13
system.

I've got TCP stream connections working flawlessly, it is
just the UDP traffic that seems to barf.

Does this strike a familiar note with anyone?

When I run wireshark on the KVM host machine, I see
NFS packets retransmitting a lot and I also see ICMP
messages about Destination Unreachable, Fragmentation
Needed. (I don't know what any of it means though :-).

This is an intel motherboard with these ethernets:
04:00.0 Ethernet controller: Intel Corporation 80003ES2LAN Gigabit Ethernet Controller (Copper) (rev 01)
04:00.1 Ethernet controller: Intel Corporation 80003ES2LAN Gigabit Ethernet Controller (Copper) (rev 01)