I use TYPO3 7.6.X with news extension.
I have a category "Events" that contains many Newsitems.
When I add a newsitem in this category "Events" it take a lot of time.
I did a System environment check with the install tools and all is Ok (green).
Also, nothing special in the Chrome network Tool.
What can it be like cause ?
There is likely a significant impact from reference indexing, especially if you use IRRE relations from within your records, or use heavily populated references.
You can use this extension that I created as a way to defer reference indexing from occurring on-the-fly to being processed as a queue (by a cronjob).
https://github.com/NamelessCoder/asynchronous_reference_indexing
For a TYPO3 7.6 compatible version you have to download/require the 1.2.0 version which is the last one to support 7.6:
https://github.com/NamelessCoder/asynchronous_reference_indexing/tree/1.2.0
If you composer require namelesscoder/asynchronous-reference-indexing and use composer to install TYPO3 you would get the right version automatically but for manual installs you have to select it yourself.
The "Background" section of the README.md explains a bit more in detail about why reference indexing sometimes becomes a very narrow bottleneck for performance in backend.