[BUG] Mars import took too long

[chaokunyang]

Describe the bug
When import mars first time, it took about 4~5 seconds which is pretty time-consuming for users

To Reproduce
To help us reproducing this bug, please provide information below:

1. Your Python version: 3.7.9
2. The version of Mars you use: master
3. Versions of crucial packages, such as numpy, scipy and pandas
4. Full stack of the error.
5. Minimized code to reproduce the error.

Expected behavior
import mars should take less than 1 second, just like pandas:

Additional context
Add any other context about the problem here.

[wjsi]

Please run

python -X importtime -c "import mars"

and upload its outputs as a log file here.

[chaokunyang]

Please run

python -X importtime -c "import mars"

and upload its outputs as a log file here.

import.log