In "Manage Jenkins", the plugin manager (aka update center) is rather slow. Slow is about 3 to 6 seconds on my windows laptop.
The http requests of the plugin manager are mostly the slowest of all, as can be seen in the joined screenshot of the monitoring plugin.
The screenshot also shows that those http requests have a high cpu usage.

The cause of the issue is that for each plugin, the UpdateSite.getPlugin(String) and getData() methods read and parse all the plugins data from the "updates/default.json" file each time.
So the more plugins are available, the slower it is.

I will submit a pull request.