The 150x speed is usually the read speed. The 150x speed is also usually the test read speed obtained from a large block of file whose size is the optimal for read transfer for this CF.
eg, If you are reading 1024 x 1KB size files vs read 1 x 1MB file, the 1MB file will be read or written much faster.
Further, write speed is usually lower and the card reader do play a part.