What could possibly slow a bike down on a slight decline? (it isnt the brakes)
 

This is my new bike that was just built up. The bike seems to be tougher to get moving in the higher gears on start out. Initially, I attributed it to smaller crank and different gearing.

Today I noticed that on slight declines I actually lose a little speed. On flats I lose quite a bit of speed. This occurs when I am not pedaling.

Could this be a bottom bracket not being installed correctly? Or The cassette binding on the rear hub?

This bike was just put together from scratch.

Check to see if the rim is rubbing on the brakes.

also you might want to check the adjustment of the wheel bearings. Maybe they are overtightened.

check the title :)

Oh...

Probably not the bottom bracket, since you're not pedaling. It could be the cassette binding, as someone suggested, but it doesn't seem too likely. I would check the bearings, and make sure the wheel isn't rubbing anywhere.
If you have knobby tires, they could be responsible.

Can't possibly the BB. If you really reef the wheels' QR skewers down way too tight, they can put pressure on the hub bearings. Beyond that, rolling resistance - tire type as mentioned above, or they're underinflated.

To see if it's the freehub, get into a coasting descent, get a sense of the deceleration, then start pedaling just fast/hard enough to keep the freehub from ratcheting, but not enough to actually propel the bike. If that seems to lessen the deceleration, then indeed the freehub may be dragging you down.

Or lift the rear wheel, spin it forward, and see if the chain starts moving. That would show that the cassette body is tight to the hub.

Compared to...?

Different tires, tight freewheel mechanism, sealed bearing dirt seals not worn in yet, less aerodynamic position (taller, wider)...etc.

If it occurs while you are NOT pedaling it is not the bottom bracket.

If you're not fighting a headwind then I suspect either the wheel bearings or possibly the seals. Those can be fixed by fine tuning the bearing preload and by lubing the seals with a bit of oil or silicone spray.

But the most likely culprit is the tires. Tires with a heavy and coarse tread face have more rolling resistance than tires with a thin "slick" tread. And tires with stiff sidewalls seem to suffer from increased rolling resistance as well from my experience. And if you have tires with knobby or very open tread then there's your problem. The key is that rubber has something called hysterysis. That means it is a lousy spring. It soaks up energy and turns it into heat. The energy enters into the rubber when you flex it and only part of it comes back out. So the more rubber in the spots that flex the most means less power goes into riding. Or in this case your tire choice can easily be the difference between slowing down or speeding up on this slight slope. And if you're not inflating your tires enough then again you're promoting rubber flex and energy loss.

Wheel bearings or wheel bearing adjustment.

you could check if it could be your rear wheel by putting it in a stand pedal the cranks hard and see if the rear wheel drives the cranks forward. If so there is a problem with the rear wheel freehub

so stop the pedaling action and see if the hub moves the chain and the crank?

Just keep in mind that even a really nice condition freehub will propel the chain and cranks forward a little if the BB is nice and free. If you do this the way to tell is to just stop the pedalling and then hold the cassete steady with the other hand and let go of the crank. If there's a lot of freehub drag you'll feel it right there.

But you don't even need to do that. It would be best to isolate the freehub from the chain and crank arm to better evaluate the condition of it. Pull the wheel out of the frame and gently turn the freehub backwards and see how much torque it takes to do so. That way any resistance in the cranks isn't part of the equation. The amount of torque needed to turn the freehub or freewheel is what the wheel would transfer to the cranks when riding and the amount of drag that would be slowing the bike down. It's all about separating the parts of each system so you can evaluate the condition of the various parts to see where the trouble truly is.

By the way, don't ignore the front wheel in all this testing. A dragging brake or binding hub will slow the bike down every bit as effectively as a similar issue at the rear.