[T-hawk]

Matching speeds to the target object isn't all that difficult, right. The problem is the travel time becomes enormous to a distant target object, because it's going so slow comparatively.

Pluto's orbital period is 248 years. If you aim to match its velocity when you arrive, the journey takes a significant fraction of those 248 years. Compare Mars, where the orbital period is 22 months, and a Hohmann transfer orbit takes 9. A similar trajectory to Pluto would be on the order of 80 to 90 years.

It's not just about the magnitude of the speed vector, it's also the direction. To match and orbit Pluto, you need that 4.7 km/sec to be tangent to the sun. You need to start out slow enough so that your radial velocity can fall to zero by the time you get there. You can't apply more radial velocity to get there faster, either by launch or slingshot, if there isn't equivalent fuel or mass to decelerate you; you'll overshoot as New Horizons did.

[burfog]

We now have ion thrusters, which are great for when you need a huge velocity change but can take your sweet time to do it.