13" and 15" MacBook Pros Have a Slower SATA Interface
The slower SATA interface is unlikely to affect the bulk of users as even the fastest traditional hard drives are unable to saturate even the 1.5 Gbit interfaces. However, if you are planning on buying a fast Solid State Drive (SSD), it could affect the drive's performance. The downgrade of the interface in the new MacBook Pro has also been confirmed in early benchmarks using a fast enough SSD. Forum user fpnc provides an excellent summary of the findings so far:
1.) It appears nearly certain that the new 13" and 15" MacBook Pros are all reporting a SATA interface running at 1.5Gb and not the faster 3.0Gb rate that has been in pretty common use for the last few years. These new models have the Secure Digital (SD) slot and also appear to have redesigned motherboards.
2.) Those who are using standard hard disk drives will probably see no difference in performance. If that is you, you can stop reading now.
3.) Benchmarks on FAST solid-state drives (SSDs) are showing a decrease in RAW disk i/o transfer rates on these same systems (in comparison to the previous generation MacBook Pros and MacBooks).
4.) The largest differences in the benchmark results seem to be in large, sequential disk READS (one of the traditional strengths with SSDs).
5.) To the best of my knowledge, no one has done any test with REAL-WORLD operations to show that the user experience (i.e. "performance") will be decreased with the 1.5Gb SATA interface. That is to say that thus far we've only seen benchmarks done with RAW disk i/o benchmarking tools.
6.) No one really knows why this has been done and no one knows whether it can be fixed with a software/firmware update (it may or may not be able to be fixed).