Moving at free-flow and moving faster than without the freeway are two very, very different things.
Most of the traffic savings from the 413 are expected to be diverted traffic off of arterials roads in Peel and York Regions, where traffic is much slower speed and gets caught at large, long signals. Even if you are traveling at an average of 40km/h on the 413 (a very congested average speed for a freeway), you will still be travelling significantly faster than driving all the way down Mississauga Road to the 401 from Mayfield, for example, or all the way across Major Mackenzie from the 427.
The 413 is not expected to make major differences on congestion levels on existing freeways in the region (albeit small differences), but will offer more direct paths for driver which will create time savings for drivers using the highway itself.
The big travel time savings on existing corridors needs widening of the existing corridors themselves, a la the 401 widening through Mississauga.