Rate limiting/policing can be used using the "police input" command on the ASA's outside interface.
Here is a good example page http://supportforums.cisco.com/docs/DOC-1230
Keep in mind that if you police inbound you are not achieving much because your pipe is already full. It would be best to policy as close to the source as possible.
It will help though as TCP will converge to a speed closer to what your policing rate is.
I hope it helps.
PK