If you connect your SSD with USB to your computer you are limited by the USB bandwidth.
A Samsung 840 SSD can do ca. 500MB/sec read, the USB 2.0 has a theoretical limit of only 60MB/sec but practically it can only do about 30MB/sec. This is likely your problem, USB2 is just too slow.
With an USB3 port (and USB3 cable and USB3 capable casing) you should be able to utilize most of the speed of your SSD even externally. (or with an eSATA port, or something other which is faster than USB2.)
Also if your drive is formated to NTFS it will be significantly slower on linux than on windows, because the linux NTFS driver is a slower one. This is because the NTFS file system is proprietary, and Microsoft didn't release the specifications of it, so the guys who wrote the linux driver had to reverse-engineer the file system. If you swtich to a linux native file system, like ext4, you will likely get better performance.