I add this little 'story' here as a warning, that may be helpful for performance troubleshooting.
Some time ago I bought used MINI ITX motherboard with Intel T8100 CPU and 965GM chipset, it came along with used 2 x 512MB RAM 667MHz DDR2. I already had 2 x 2GB RAM 800Mhz DDR2 from my older box, so the first thing I did after assembling all the parts was upgrade the amount of RAM.
The system worked well, 50+ days of uptime until power loss from time to time, it has ZFS mirror on 2 x 2TB drives. But I 'found' some strange performance problems.
So I started to look for the cause of the problem.
First I thought its slow because I have 80% filled ZFS, but after upgrade to 8.2-STABLE and various performance improvements it should not be a problem.
Then I thought that CPU may be overheating ... but it wasnt.
My 2TB disks are LOW POWER Seagate's so I thought that maybe their random access time is so low that its because of that, nope.
I also thought that it may be because of maybe broken 8GB CompactFlash card that is used for hte base system, nope.
At that point I did not had any clues what to check more.
But I thought, maybe I will also check memory ...
I found some nice simple memory allocation benchmark called
ebizzy (nor in ports) and run
# ebizzy -s 4096 to check the speed, compared with memory allocation speed on somparable laptop ... and VIOLA! Thats the problem.
I have just 'degraded' memory from 4GB 800MHz to 1GB 667Mhz and now allocations in
ebizzy raised from
11462 records/s into whooping
2347823 recors/s 
Also ffmpeg performance improved from 0.2 FPS into ... 40 FPS
Its because 965GM chipset maximum frequency is 667 MHz.
You have been warned ]:->