As @crw already described, the navigation panel is loading way to long.
https://forum.xwiki.org/t/your-xwiki-usability-pain-points/440/13
I couldn’t find the new topic he wanted to open so I do it now because it really bothers me.
Maybe we can discuss possible workarounds or improvements.
Greetings
Timo
Thanks for opening this topic. I did not follow up on my last post in that thread.
Well, is there any progress?
Maybe an already opened JIRA ticket?
Hello? 
Hi Timo and all. There’s no contributor/committer that I know of working on the navigation panel ATM. Re JIRA feel free to open one (there might already be one I don’t know).
Right now there are also other issues with the navigation panel, such as the ability to blacklist entries (which is a high listed feature request). The reality is that there are very few people available to work on this right now. I wish we had more contributions.
So if you want to make this topic progress fast, I see only 2 solutions:
Barring that, I fear that no major progress will be made till next year, hopefully before April.
Note that a workaround is to not use the Navigation panel and use a manual static tree instead.