I have a large dashboard (around 15 tabs). Upon loading the Dashboard I notice a very long time before I can get it working. As it is quite annoying, I made a Lighthouse report on it (attached) and - as you can see - the problems of the slow operation are all out of my control. Perfomance index: 19%. It looks like the dashboard palette itself has bad performance, but I hope I am wrong and that I can do something to improve it.
Is there an iFrame embedded in there? Also, do you have a load of ui_template code?
Certainly there are some pretty poor web practices built into the core Dashboard code. Not really anyone's fault, it is simply showing its age. For example, it contains non-passive event listeners which cause blocking events.
But you certainly have some very heavy stuff in there with 1.5s of blocked time vs 240ms on my simple Dashboard. and over 6 seconds to first content paint vs .3s on mine.
Unfortunately, Dashboard is based on Angular v1 which is really past its sell-by date.