In my experience, a "bad" ride like you describe is either a headwind, or something mechanical, like your brake is rubbing. A strong headwind could be why the 17 mph section (tailwind at the time) has become 10 mph with a lot more effort. A rubbing brake is easy to check for. Lift up your bike, and try to spin your wheel, if it stops very quickly, it could be your brake.
'02 Eddy Merckx Fuga, Selle An Atomica
'85 Eddy Merckx Professional, Selle An Atomica
'10 Soma Double Cross DC, Selle An Atomica
Slacker on wheels.