ぼちぼち、リリースノートを読み始め。
以下はamd64のリリースノートから、カーネル系。

・HZのデフォルト値が変わった。割り込み系とか時間の粒度とか、よりよくなるかも。
The default HZ parameter (which controls various kernel timers) has been increased
from 100 to 1000 on the i386 and ia64. It has been reduced from 1024 to 1000
on the amd64 to reduce synchronization effects with other system clocks.

・panic/強制リブート時の立ち直りを早くする工夫。
Recomputing the summary information for “dirty” UFS and UFS2 file systems is
no longer done at mount time, but is now done by background fsck(8). This change
improves the startup speed when mounting large file systems after a crash.
The prior behavior can be restored by setting the vfs.ffs.compute_summary_at_mount
sysctl variable to a non-zero value. [MERGED]

・NFSサーバの虫がとれたらしい。
A kernel panic in the NFS server has been fixed. More details can be found in errata
note FreeBSD-EN-05:01.nfs. [MERGED]