26 does not = faster acceleration unless that wheel is LIGHTER. Most MTB wheels are wider and heavier than their roadie 700c counterparts. Mass matters and lighter wheels will accelerate faster. Larger, lighter wheels will also slow down more quickly because of mechanical advantage (think longer lever/distance to hub) when using rim brakes.

Think if the MTB wheelset at 2100 grams shod with 600 gram 'light' tires while a 'beefy' set of 700c wheels weighs 1800 grams with 700x28c tire coming in at 300 grams. Total package on the MTB is 3300 grams while the 700c bike is 2400 grams. The difference of 900 grams or 100 grams shy of 1lb. I'm using my cyclocross bike set up for comparison which is heavy by roadie standards.

Mountain bikes can feel more responsive in traffic because of the leverage at the wider handlebars and higher relative bottom bracket height. Also, most 26 mountain bikes o have a gearing advantage with 42/32/22 chainrings and 34 teeth on the largest rear cog compared to a road bike's 50/34 or 52/39, unless you have a triple, with usually 25-26 teeth on the largest rear cog.

So, you can build a very nimble and quick bike with 26'' wheels, you just have to choose your components very carefully. I think my son's Cannondale M900 with its very light frame would be great start (... drifting off into planning mode... )