So with all this JavaScript code and high end JS based web development going on, pages need to render and the heavier the code and the slower the server and computer accessing the page, the longer the wait time for the page to render. For most modern sites and computers, this is not an issue but still, it can take some time.
John Mueller of Google was asked if there is a timeout period for when GoogleBot will not wait any longer for the page to render.
John said on Twitter no, there is no fixed waiting period or timeout period for this. He said "there's no fixed timeout since rendering time can be quite different with cached resources." "I'd double-check with the mobile-friendly test; if it works there, it should work for indexing too," he added.
Here are those tweets:
There's no fixed timeout since rendering time can be quite different with cached resources. I'd double-check with the mobile-friendly test; if it works there, it should work for indexing too.
— John ☆.o(≧▽≦)o.☆ (@JohnMu) May 24, 2018
Of course, you want to make sure everything loads super fast, not just for the spiders but also for your end users.
Forum discussion at Twitter.