Your battery's charge can be seen similarly to a can of gas, when you release gas from a can you release pressure, similarly when you use charge in your battery you release pressure. As the pressure drops, inside the batt you need to turn the valve open more to keep up the same flow of electrons, if you are starting with the valve already wide open, then of course you are going to notice a drop off in power. Going back to the can of gas, now let's attach a pipe to the can, the diameter of the pipe will affect the maximum rate that pressure can flow, in the batts this is called max continuous amp rating. Like I said previously, I don't use 26650s but I believe there are 18650's that can put out higher amps continuously than 26650s are able to do safely. Back to the can of gas, if your can is larger & has a thin diameter pipe, then it will push out gas for longer but at a slower rate than my smaller can of gas with a larger diameter pipe. The mah rating may be higher for 26650s (although this may not be the case) but they have "thinner pipes", some 18650's are much better at providing the power needed because they have "larger pipes". When using FastTech, always pay the $2.50 or $3 for expedited shipping, you'll get your parcel within 2 weeks & there is a expedited option that allows batteries to be shipped (Pro tip - ship batteries this way without other items).