Thanks.

It's Intel i7-4770 CPU with 8MB of cache and it was already running at -t 4 setting automatically. (At 50% CPU utilization.)

And like I said it's my old PC, so nothing else runs on it.

So, OK having added -t 6 command line now it runs at 75% CPU utilization.

I wish there was a setting to rev it up all the way to 90% or so?