There are two reasons for the slow stuff in the swimsets

a. When doing drills and practicing technique it is better to swim slowly as this unmasks technique errors. Swimming quickly hides errors and so does not encourage good technique, which in turn means your technique will not improve, which will in turn limit any improvement in speed. In other words you need to swim slowly in order to swim quickly.

b. Some sets are designed to improve your aerobic system, now I'm sure that I don't need to explain that if you go at it at warp factor 10 then you are actually going anaerobic which really defeats the point of the set and will affect you ability to do longer swims.

Concentrate on trying to decrease you stroke count and slow down the stroke rate. In the first instance aim for 1 body length per stroke, which equates to between 12 - 15 per 25 meters. Once there, well Olympic swimmers can cover 50 meters in less than 15 strokes, Of course to reduce you're stroke count you need good technique and for that you go slowly.

Simply to encourage front quadrant swimming, i.e keeping your arms mainly in front, something which will improve your swimming.

To make your technique better. Drills isolate component parts of the stroke and allow you to work on it until it's ingrained into your subconscious and becomes automatic. If you are unsure as to why you are doing a certain drill or what the drill is working on the ask. You don't want to ingrain the wrong thing do you?