Long rendering time caused by the sidebar

[oscarpenelo]

• I confirm that this is an issue rather than a question.

Bug report

When building a project that contains a complex sidebar (many items) the rendering takes 35min to complete. Compiling the same project without the sidebar takes 2 minutes.

Steps to reproduce

AWS documentation in markdown is being used as a build test (https://github.com/awsdocs)

Run in the terminal

#clone repo with a complex sidebar, you can see the sidebar here: https://github.com/oscarpenelo/vuepress-test/blob/master/docs/src/sidebar.json
git clone https://github.com/oscarpenelo/vuepress-test.git
#install dependencies
cd vuepress-test/docs && npm install
#build without sidebar and check the duration. Internal command: sidebar=false node --max_old_space_size=8192 ./node_modules/vuepress/cli.js build src
time npm run buildwithoutsidebar
#output: npm run buildwithoutsidebar  113,68s user 15,61s system 104% cpu 2:03,85 total

#build with sidebar and check the duration. Internal command: sidebar=true node --max_old_space_size=8192 ./node_modules/vuepress/cli.js build src
time npm run buildwithsidebar
#output: npm run buildwithsidebar  1952.99s user 42.48s system 95% cpu 34:55.82 total

What is expected?

A compilation time under 5 minutes

What is actually happening?

20 times longer duration with sidebar than without (35minutes vs 2minutes)

Other relevant information

• Output of npx vuepress info in my VuePress project:
  Environment Info:

  System:
  OS: macOS 10.15.4
  CPU: (4) x64 Intel(R) Core(TM) i5-7600K CPU @ 3.80GHz
  Binaries:
  Node: 12.16.1 - /usr/local/bin/node
  Yarn: Not Found
  npm: 6.14.8 - ~/tttt/vuepress-test/docs/node_modules/.bin/npm
  Browsers:
  Chrome: 85.0.4183.121
  Edge: Not Found
  Firefox: 81.0
  Safari: 13.1
  npmPackages:
  @vuepress/core: 1.6.0
  @vuepress/theme-default: 1.6.0
  vuepress: ^1.3.0 => 1.6.0
  npmGlobalPackages:
  vuepress: Not Found