Replacing thick wheels with thinner ones My heavy bike (~15 kg) has thick wheels, the rim width of each is about 3 cm. Accordingly, the tires are also thick (~4.5 cm). Unless the tires are fully pumped up, the bike is difficult to ride and is considerably slow (due to significant traction).

I am thinking of replacing the wheels with thinner ones. Is this possible in principle? If so, what issues have to be considered before doing that?
Your problem is that you're riding a BMX bike.  While you can certainly go narrower than 1.75, you can't go much narrower -- maybe to 1.5" (3.8 cm) with the same ISO 406 wheels.  You might be able to switch to a slightly larger ISO 451 rim and get down to 1-1/8" (2.8cm) width, but it would be quite expensive to swap the rims and spokes, and the tires might not fit your frame anyway.

If rolling friction is an issue for you you'd be advised to switch to an "adult" bike.  Just about any "adult" bike, new or used, will roll significantly easier than the BMX bike -- they're designed for hot-dogging, not riding any distance.