There are different notions of "fast" as well. Theano might be faster on a single GPU (not actually sure if this is still true), but TensorFlow can be distribut

There are different notions of "fast" as well. Theano might be faster on a single GPU (not actually sure if this is still true), but TensorFlow can be distributed to multiple GPUs in one machine or even across several machines. Several groups have reported speedups that are near-linear in the number of GPUs you distribute across. This can make TensorFlow orders of magnitude faster than Theano.