Extreme slowness loading Page or Design in Edit mode

Hello,

I have a huge problem when I work in the CMS (8.0.21 since 2 weeks and I already had this problem before : when working with 7.0.103). I have to wait about one minute to load the page I'd like to work in. In Chrome or IE, I don't receive any information about that but in Mozilla, I got a message telling me that 'a script doesn't respond : it may be occupied or doesn't work... '

The script which is problematic is .../CMSAdminControls/CKEditor/ckeditor.js:698 (the number changes following the moment I stop the script).

I don't know what to do to improve this slowness. Do I have to change the text editor (can I do that?)? Is there something missing somewhere to call this script ?

Can someone help me because it's really disagreeable to work in such conditions ?

Sorry for my poor english,

All the best,