[Query] Using ctx=gpu performance difference. Is this expected?

Hi. Thanks for the amazing repository.

I installed using `-DUSE_CUDA` option and then tried the example here (https://github.com/zhreshold/decord/blob/master/examples/video_loader.ipynb). I averaged over ten runs using %time Walltime output. The statement I timed was

`vl = de.VideoLoader(videos, ctx=ctx, shape=shape, interval=interval, skip=skip, shuffle=0)`

| cpu | gpu |
|-----|-----|
| 53 | 40 |

I also tried across various shuffle strategies, but nearly all of them were the same when the same device is used.

Wondering if this is what is expected.