Your SSD needs some maintenance from time to time to perform well on the long run. After some use, it gets slower and slower if it isn’t told which parts of it are still in use and which hold only garbage. This is due to the internal workings of the disk that I won’t go into here. If you’re interested in the gory details, please head on to Wikipedia: TRIM.
On Ubuntu, you can refresh your SSD with the fstrim command:
sudo fstrim -v /
This applies the optimizations to the root filesystem. If there are other filesystems that are mounted from an SSD, replace the single slash by the path to the mount point(s).
On my laptop, which is equipped with a 250 GB Samsung SSD 840 drive, you get the following results:
After trimming, the SSD is not as fast as when it was new, but the average reading transfer rate improved by 15%.