gpuarray.dot() works too slow at the first calling

I found it will cost much time when the first calling of **gpuarray.dot()**.
Here is my code:
```
...
# the first time calling
start.record()
# res_gpu = gpuarray.dot(coef_gpu, image_gpu)
gpuarray.dot(coef_gpu, image_gpu)
end.record()
end.synchronize()
secs = start.time_till(end)
print("\ntime cost: {:.3f}ms\n".format(secs)) # time cost: 813.931ms

# the second time calling
start.record()
# res_gpu = gpuarray.dot(coef_gpu, image_gpu)
gpuarray.dot(coef_gpu, image_gpu)
end.record()
end.synchronize()
secs = start.time_till(end)
print("\ntime cost: {:.3f}ms\n".format(secs)) # time cost: 0.056ms
...
```
Why it will happen? And how can I solve the problem?