I don't know if it happens to others, an Illustrator on a Windows machine "I would say performing, quite a lot" but this problem happens to me I think at least from the CC2018 / 2019/2020 version.

I start the software, open a new empty document, or open an existing document (even of small, insignificant dimensions) the document takes a long time to open (it is not immediate).
If I open a second document it opens in the right time, then it works as it should.
The same thing happens if I close Illustrator and restart it.

But before it DID NOT HAPPEN with older versions of Ai.