Macbook Pro Early 2011 SSD very slow when in main hardrive bay but very good via USB
The problem seems to be only with that particular SSD and Mac OS X, because it works with other SSDs. My feeling is that garbage collection or TRIM are to be blame in this case. TRIM is disabled by Apple for third party drives and ADATA SSD I had didn't have any inbuilt garbage collection. 

Bought a Samsung EVO now, it has inbuilt garbage collection and doesn't necessarily need TRIM. If you have an SSD which can't do garbage collection, there are options to enable TRIM on Mac OS X, look for them and enable TRIM.