Sure. If you take something as simple as the number of supply days per period, by taking one day out of the five-day week, you've reduced the number of days the House sits by 20%. The number of supply days per period is set down in the Standing Orders, and it's fixed. That increases proportionally the number of supply days in each period relative to government business. That's one example.
For private members' business, you would lose an hour of private members' business. If you didn't make it up somewhere else, you'd lose that. For bills and notices, a bill can only be read once in a given day, so if you lose a day in a week, that delays the options. It reduces the options for the government.
For notice periods, if you take a day out, what do you do with that day? Do you allow it to continue to be a day that's valid for notice purposes or not? That's something to consider. This is where two sittings in a day kind of compensates for that. If you still have five sittings in a week, you could still accomplish a measure of what you would have accomplished or could have accomplished in a five-day week. Those are some examples, and there are of course many others.