Sure, an SMX can also switch between warps in the manner similar to hyperthreading on x86 but it does not mean it executes a single warp at a time. Consider Tesla K40, a GK110 with 15 SMXs. It runs 750Mhz and has peak performance of 4.29 Tflops. If each SMX could only execute a warp at a time it could get, at most, 15(number of smx) x 32(warp width) x 750M(frequency) x 2(two flops per FMA) = 720Gflops.