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Good advice on tyres, but do not forget the bearrings! Is the hubs greased and adjusted? Sometimes I stumble upon bikes where a wheel has overtightened hubs so rotating the wheels to watch it spin is almost impossible.
BB. Is the Bottombracket ok, spinning freely? It helps to let the chain hang loose (not on the front chainwheel) when you try but best is actually to remowe the crank arms and feel the axel, if it rotates freely.
You can place a screwdriver on the axel of all three mentioned and rest your ear on the other end. That way you can "listen" to the hub, listen for grinding sounds.
Pedals. Are they spinning free or do they need to be serviced or replaced? A lot of energy can be lost "fighting" bad pedals.
Wheels.Are the wheels straight and true? Rear wheel aligned (if horizontal dropouts) like it should? Brakepads rubbing on wheels that need straightening or rubbing becouse arms should be remowed and greased or binding cables that should be repoaced?
Luggage, chlotes. Is your luggage, chlotes or body catching the wind like a sail? Need to button up your jacket (or take it off) use narrower luggage or loose weight?
To ask if a Dahon is slow is like asking if french peopel are slow.. It depends.
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