In fact it is the default. So if you don't get that speedup, you are turning it off with -I or something that implies it (eg -a).

Look in the manpage. There are a lot of options. And some of them imply other options. I found in particular that I never use the -a option because it implies a ton of flags, one or two of which I don't want, and there is no way which I saw to override that easily. So have scripts that use a gazillion flags instead...

Cheers,
Ben