That's a really good idea. We could try that with my friend's wheels--from her Salsa.
Printable View
I'd say, combo of things, already mentioned above. Wheels (maybe) but more likely the tires and maybe the bottom bracket. Try another wheelset that has different tires on it. I had Kenda Negevals and they suck. Most versions are thick heavy tires. Yea they stick but they are like riding a bike with thick strips of sorbathane wrapped around the wheels. It's not the weight of the tires, that's a small factor. It's the materials/design. Those tires just suck. I'm riding Tioga Psycho genius tires now, not light weight but they roll really well. Wheels can also just be loose. Squeeze a few spokes together and compare the tension to other wheelsets. A really loose wheelset will be VERY different.
You have already eliminated the hub bearings, as they spin down well, and heavy brake drag. So check that bottom bracket, it's easy to compare two bikes with the chains removed. The crank should spin without crunchy noises and smoothly.
If everything checks, and different wheelset makes no difference, I'd say sell it and move on, the geometry just isn't working for you.
Oh, I am already moving on...my Salsa El Mariachi 3 should be in my hot little hands sometime next week! :D The Cannondale will be going on to my 11 year old DS. We figure he will fit it for a year, but in its current operational state he's going to be loathe to use it. Once he outgrows it we'll likely scour eBay for a used frame that we can build-up...he can learn some wrenching with this, too. Or a used bike with better components to begin with.
Yesterday I had the opportunity to test an El Mariachi 2 during a Salsa demo, yesterday. I did one loop of some new trails on that bike. The handling was very different, in part due to handlebars that are way wide for me. It didn't feel any different on downhills, but when I did 2 more loops on my current bike I realized that there were a lot of uphills that I totally did not feel on the Salsa. My El Mar won't be quite as zippy as the one I demo'd (I think there's about a $500 difference in components, which is pretty huge), but I shouldn't be losing momentum as soon as a trail flattens and starts to rise, anymore.
That's something I had forgotten to mention. During a relay race this Summer I was just behind a pro endurance racer, Danielle Musto (who is, BTW, the NICEST pro athlete you could imagine. She shuts all the stereotypes down) and was actually gaining on her on a grassy downhill section. Afterwards we were laughing at how I sunk like a stone on the Cannondale, but as soon as we leveled-out she and her Ti El Mariachi were gone. So, yeah, that makes it seem unlikely that it's an issue of hubs or brakes rubbing, too.
congrats on a new bike!!