VuePress2 beta.50 has serious performance issue comparing with beta49

[Mister-Hope]

VuePress2 is already far slower than VitePress due to it's design structure, beta50 is even less worse than beta.49.

I did no changes with actual js code when migrating to esm and compatable with vuepress2 beta.50 in my project, but while beta49 took less than 10min building docs (with 4096 max_old_space):

See: https://github.com/vuepress-theme-hope/vuepress-theme-hope/runs/8051793959?check_suite_focus=true

Beta 50 even cannot run with 6192 max_old_space, and the built time is more than 40 minutes with a max_old_space of 8192:

https://github.com/vuepress-theme-hope/vuepress-theme-hope/runs/8056946496?check_suite_focus=true

I think we will need some investigation on performance between 2 versions in large documentation sites.

[Mister-Hope]

Some improvements are made in ea0a2b2 then published in beta.51

But large project can still get a performance slow down up to 300%

@meteorlxy made some investigation, as newer version of vue-router is one of the preformance reasons.