I did my first 2,000 miles in my road bike with flats before I swapped to spdsl. Didn't notice any real difference in speed or distances covered.I tried that on Chitty (my ancient MTB) when on a three day ride of the south coast and found it really hard going. I am amazed at how much I rely on the "Clippage". I ended up stopping in Andover to buy additional pedals, shoes and clips.