title: keras predict is very slow
body: <p>I am working on a reinforcement learning task and decided to use keras NN model for Q value approximation. The approach is common: after each action the reward is stored in a memory replay array, then I take random sample from it and fit the model with new data <code>state-action =&gt; reward+predicted_Q</code>(more details <a href="http://outlace.com/Reinforcement-Learning-Part-3/" rel="noreferrer">here</a>). In order to do the training the Q value has to be predicted for each item in the training set.</p>
  
  <p>The script is running very slow so I started investigating. Profiling shows that 56,87% of cumulative time is taken by <strong>_predict_loop</strong> method:
  <a href="https://i.stack.imgur.com/0f043.png" rel="noreferrer"><img src="https://i.stack.imgur.com/0f043.png" alt="enter image description here"></a>
  And it looks strange, cause prediction is just a one-way propagation. Just a one-time multiplication of set of numbers. The model I am using is very simple: 8 inputs, 5 nodes on hidden layer, 1 output.</p>
  
  <p>I have installed and configured CUDA, run few example tests and it shows that GPU is used, also I can see huge load of GPU. When I run my code - there is a message: "Using gpu device 0: GeForce GT 730" but I can see that <strong>GPU load is really low</strong>(about 10%).</p>
  
  <p>Is it normal for predict function to take so much time? Is there a way to use GPU for this computation?</p>