Quickly searching for a note

Background: To tie Perforce changelists to Git commits, I add a note to 
a commit with the form "P4@123456".  Later, I use the note to sync down 
the closest Perforce changelist matching the Git commit.

I search for these notes by getting a list of revisions:

         git rev-list --max-count=1000

I iterate those revisions and run git show and grep on each:

         git show -s --format=%N%n%s --show-notes=p4notes COMMIT

For short runs, this isn't so bad.  For longer runs of commits (I just 
walked through approximately 100), it takes a long time. Running 'git 
show' is costing me about 7/10 of second, presumably because I am on 
Windows.

Is there a faster way to do this?

Thanks.

Josh