Hi @Andrea_Bozzano ,
welcome to the Kibana community.
Would you mind sharing a HAR file that captures all that slow requests? Make sure to remove anything that may be security sensitive.
Also, is this happening only in one browser (i.e. Firefox) or in any browser?
The core.entry.js file is fetched the first time a space is loaded, and all subsequent dashboard loaded should not fetch it again. Also, when changing space, a full reload is requested, but the core.entry.js file should be picked up by the browser cache.
I just did a test on a 7.12 stack and the file was correctly cached after changing spaces. Also, the file is less than 100kb which should not take so long to load compared to other resources in a dashboard, like data fetching for multiple panels for instance.
Are you sure the kibana server is not delaying some requests for other reasons, like lookup fields for an input control or maybe running out of disk space on the elasticsearch node which may queue many requests