Townie gearing suddenly too easy
I am not sure if I got it wrong, but to me it seems that what you describe as a problem is in fact a solution.

It is expected to get a SUBSTANTIALLY faster bike if you replace a tire, because of tire quality, rubber compound, thread pattern, pressure, etc.
1.5 size, comparing to 2.0, is a specially much more efficient tire measure, which would for sure make the difference you mentioned.

If you are getting "the result" (bicycle is moving) with less effort, and the old gearing seems inadequate, I would suggest you to update the gearing, for example, getting a bigger chainring (not TOO bigger), and keep going faster.

If you don't want to spend, you can just enjoy less effort for the same speed.

Keep in mind wider tires might be more comfortable due to cushion, and more resistant to impact-induced flat tires, because they have larger air volumes. But except if you don't feel the bike right, consider to keep the narrow tire (perhaps exchange also the front one) and enjoy a new level of cruise speed at lower efforts.