1.5 minutes for first train, get off that train 1.5 minutes wait for second train. You can now get to any point on an XY grid with ~3 minutes of wait time. 1.5 minutes works if your only going down one line but that's not a huge number of stops.

Anything less than 5-10 minutes is generally considered to be "frequent service", where it's no longer necessary to worry about hitting a specific train, since the next train will be along fairly quickly regardless.

There is a large quality of life difference based on maximum trip time.

If I need to make 2 trains each way with an max of 15 minutes of wait time each. Then I need to add 1 hour of padding to cover that worst case over the day or risk being late. Further, if anything happens that can go well past 1h of just waiting. Cut that to a daily 12 minutes 'worse case' makes a massive difference.

That could also be improved with timed connections.

Those are hard to pull off across a full XY grid as soon as something happens like a line is single tracking or your grid is missing a stations it's a mess. You also really want some slack in the system for cases like single tracking if the average train is 4 cars, but you can send 6 cars trains when demand is up that makes a huge difference.

So, yea that's one way to lower the average wait time, but again just sending more trains and the system becomes more stable.