I’ve a large project with 100’s of cms items and each page has lots of collections on it - almost the limit of 20 - most are stored within dropdowns in the nav bar.
Because of this, the Designer - both the CMS and the canvas can be quite slow and laggy. I’m pretty sure it’s just down to the sheer volume of data on each page.
However, if I create a new blank page - and go to that page when editing the CMS, it’s much much quicker. I think it’s cos there’s nothing on the canvas that there’s more processor power available.
That’s it - hope it helps someone!