性能优化

[littlechao]

Please make sure to read the following list before creating a new issue:

• This issue tracker is not supposed to be used for questions on how to use visjs. It is intended to be used for bug reports and feature requests! In case you face yourself with a usage question, then post your question e.g. on stackoverflow tagged with "vis.js".
• Have you already used the github search, read the documentation and looked at the examples?
• Make sure to use the latest version of vis-network for bug reports.
• Make sure to mention which browser and OS you are using when creating a bug report.
• Please provide a minimal code example that demonstrates your issue. We recommend using jsbin for that.
• Delete this list from the actual issue.