I wonder why Torch is so slow. But, the authors did not provide the configuration of each tool in the paper or on the web.
What do you mean "so slow"? It's by far the fastest framework covered by the paper in scenarios where threads don't outnumber CPU cores.
Taken from the article itself:
"However, Torch still achieves the best performance in our experiments in which Torch has nearly 12x speed up compared with TensorFlow under 4-thread setting."
You're right that Torch is faster than TensorFlow in RNN. But Torch is slower than TesnorFlow in AlexNet and ResNet. There is a set of benchmarks for many DL approaches as found in https://github.com/soumith/convnet-benchmarks