Time in training becomes gradually slow.

[n5-suzuki]

Hi @leimao.
Thanks for your greatful work!

I'm tring to execute your code using own dataset.
I can execute training but I have a small problem.

The problem is time in training becomes gradually slow.
At first the time takes about 1 minutes per epoch, now , 550 epoch, it takes about 4 minutes per epoch.

Is this behavior correct?
Please help me..
Thanks!