Algorithms in pregel written by myself is inefficient

The code in the [sample](https://graphscope.io/docs/analytics_engine.html#writing-algorithms-in-pregel) takes 15s, which is 10 times as long as the built-in [algorithm](https://graphscope.io/docs/reference/app.html#graphscope.sssp)(1.3s).

* OS: Ubuntu 18.04.6
* Memory: 128G
* Core: 16
* Python: 3.8.6
* GraphScope: 0.17.0
* Graph File : [LiveJournal social network](https://snap.stanford.edu/data/soc-LiveJournal1.html)
* cluster_type: hosts
* num_workers: 1