@aharrison @Flole @chpalmer
I believe I have a fix - I've been running this for about 20 minutes with no lag spikes. I won't call it ideal, or even great, but it proves without a doubt that the issue is with the 2.x builds of radvd and not a network card, or vlan or lacp or insert whatever excuse issue, it's radvd.
I installed an older 1.x binary I was able to find on the freebsd packages mirror to replace the 2.17 binary. It seems to work perfectly fine (it's advertising as expected) and no more lag issues. Steps below (1.15 was the newest version I could find):
First stop radvd (disable advertisements from the GUI)
next you need to ssh into the system and go to the console
cd /usr/local/sbin
mv radvd radvd.bak
mv radvdump radvdump.bak
cd /tmp
fetch http://pkg.freebsd.org/FreeBSD:10:amd64/release_3/All/radvd-1.15.txz
tar xf radvd-1.15.txz
cd /tmp/usr/local/sbin
cp radvd* /usr/local/sbin/
restart radvd from the GUI and you should be good to go.
Hopefully someone at netgate will address this more formally. As far as I can tell at this point it breaks nothing. If you do run into issues you should have no problem backing out, just delete the radvd and move radvd.bak to radvd.