extremely slow boot of RELENG_10

[mike tancsa]

While moving a pair of ZFS disks from an *very* old motherboard to a new
one, I ran into an odd problem I had not seen before.  The box was
running just fine RELENG_10 from FreeBSD 10.1-STABLE #18 r279918 on 10yr
old MB. Boots normally and expected speeds for old hardware/CPU.  But on
the new one, as soon as the |\-/|\ starts spinning, it looks like its
1/100th of the speed it should be.  Unfortunately, we didnt have the
luxury to wait for it to boot up all the way.  We put the disks back in
the old motherboard, did the RELENG_11 upgrade which included putting in
the new boot blocks after zpool upgrade and then the disks booted just
fine in the new box.  We at first thought it was a hardware issue, but
same behaviour on another new MB, new SATA cables etc etc.

Any idea what was going on behind the scenes that might have caused it
?  Is there something about the way the old boot blocks would interact
poorly with new motherboards (in this case, a SuperMicro X11SSH-F) ?

    ---Mike