I would be surprised if a high-end smartphone in the world could out-compute a reasonably spec'ed desktop from the early 2000s (which was point at which computers began to rather consistently beat grandmasters.) The lack of CPU fan is the biggest limiting factor of all.
An iPhone 6 can do 77 GFLOPS. Deep Blue could only manage 11 GFLOPS. Now, Deep Blue had specialized VLSI chips that are hard to measure, and chess computations are going to be mostly integer, not floating-point, but the point stands that a modern phone has plenty of computing power for crushing puny meatbags at chess.