Homepage isn't cached with WP SuperCache

Hi,
I have noticed that using WP Super Cache every page, posts is cached except Homepage.
For example at the bottom of every post and page you can find, example

<!-- Dynamic page generated in 6.458 seconds. -->
<!-- Cached page generated by WP-Super-Cache on 2018-05-23 13:56:01 -->

<!-- Compression = gzip -->
<!-- super cache -->


But not on homepage. Also homepage delay is about 7-15 sec, before it even starts to loading.

In Firefox, using F12, every page/post shows this:
DOMContentLoaded: 1-3 seconds, but Homepage gets:

DOMContentLoaded: 10-20 seconds

Why such a big delay, why homepage isn’t cached.

Tnx.