The external connection you have you will always be limited by the slowest element in the system.
  • Firewire 800 is , of course rated at 800 Mb/s (0.1 GB/s)
  • According to design standards USB 3.0 can theoretically deliver 625 MB/s (0.63 GB/s) but you will never see that in actual data transfer because of the protocol overhead.
  • Thunderbolt 1 clocks in at 10 GB/s (74.5 Gb/s)
  • SATA (the drive in the enclosure uses a SATA connection is typically rated at 3.0 or 6 Gb/s (0.4 to 0.61 GB/s) According to OWC the Mercury On The Go enclosure is rated at 219 MB/s (0.22 GB/s).
  • A 7200 RPM drive will have significantly less lag time then a 5400 RPM drive and will most definitely feel faster and more responsive which is most noticeable when working with small files but makes little real difference with large files or I/O intensive tasks. An SSD has virtually NO lag time and boot files are numerous but small which is why SSDs boot so much faster.
With your present drive/enclosure the speed limit is not determined by the connection to your computer but by the speed of the SATA interface in the enclosure.

If we knew what it was we were doing, it wouldn't be called research, would it?

— Albert Einstein