Dashboard loads slowly because there are tons of authentications requests carried out while logging in, this increases your CPU usage. If your server CPU delays in serving request then this increases the load time.
Still There are many reasons for Slow functioning of WordPress Dashboard like
* High CPU Usage
* Improper Configuration of Database Cache
* Heavy Theme
Most easiest method is to deactivate all the installed plugins and check loading time by activating them one by one.
High CPU Usage
I would suggest you to delete useless plugins.
Checkout which plugin is slowing down your website , remove that plugin and install an alternative one.
Are you facing this issue after installing any plugin?
To check which plugin is using all your CPU resource, use "P3 WordPress Plugin"
here is a link for your reference.
Improper Configuration of Database Cache
If you've installed any cache plugins. disable it. If cache plugin is the problem then configure it correctly.
Switch to default theme for few time and check you dashboard theme.
If this issue is not solved try contacting your hosting provider.
Hope this helps you.
let me know your feedback.