CanalPlan timings are deliberately designed to be, by default, somewhat on the conservative side.
There are a few reasons for this of which the key one is that it is most likely to be novices who use the defaults and the last thing I want to do is to encourage someone to ruin their first boating holiday by trying something too ambitious.
Also, having it like this means you don't need to make allowances in your cruising day for shopping, filling up the water tank, clearing muck off the prop and the like.
Regular users really ought to create an account and can then keep their preferences.
It's actually a lot more optimistic than typical guide book advice. For example, Nicholson's says "add the number of miles to the number of locks and divide by three to get hours" and then suggests adding time for services and visiting pubs. That would work out at 3 miles an hour and 20 minutes a lock (so slightly faster over the water than CanalPlan on narrow canals and as slow as anything CanalPlan suggests for locks, inside or out of flights).
To compare, for the Cheshire Ring, CanalPlan suggests a total of 56 hours 20 minutes. Nicholson's formula comes out at 63 hours 40 minutes. That's a day longer.
I have, at times, considered allowing % adjustment factors for waterways - including up and down stream factors but have not yet (hint) been persuaded that it's worth the effort, nor that people will care enough to help create the data.