Any suspension you put in the back will reduce your power. Rear suspension on mountain bikes (until you get into expensive stuff but that stuff just locks out the suspension when it's flat and only kicks in when it gets bumpy) kills you power.

The best solution as some people have said is usually better or fatter tires. Schwalbe Marathon Supreme's are built with comfort as their top priority:
Marathon Supreme HS 469 | Schwalbe North America

Above a certain point fatter tires are also going to make you a little slower, but that's where I'd start - generally fatter tires also do a much better job of absorbing bumps than suspension is able to.