Just being fit and riding will increase your speed. Doing hills or intervals will do it faster.

Riding 35 km at 25 kph will take 1.4 hrs. If the terrain is fairly flat, you should be able to get to 30 kph if you so desire, a pretty good clip. That would save 10 minutes one way.

10 minutes really isn't that much time in the grand scheme of things. You can save about that much time streamlining your clothes changing and eating.

I like to go fast, so I do. But with only 10 minutes at stake, there is no reason not to take it easy and enjoy the ride if that is more your thing. The difference in effort between 25 kph (15.5 mph) and 30 kph (18.6 mph) average speed can be quite a bit.

P.S. Hills, stops, etc. really kill your average speed. If you are averaging 30 kph that implies that most of the time you are cruising at over 33 kph. Last night I took a 30-mile (48 km) ride and averaged 17.7 mph (28.5 kph). There were stops and pedestrians to deal with though. Most of my open road riding was in the neighborhood of 25 mph (40 kph). All that slowing down and speeding up really kills your time.