In the toolkit, one of the last options is, "Virtual Files".
Expand that, and left click on Config.WTF.

Then scroll up to the top of the top pane.
And expand Characters.

Drag every character down to the left pane.
I believe the default virtualization is: Config-Name-Server.WTF

This lets you change settings and have them saved on a per character basis.
You can minimize every option, or choose to minimize most, but keep view distance or whatever.
Some require a restart, so clicking options - video etc, from the log-in/password screen is a good idea.

Your slaves having lower settings will be less of a strain on the system.
Also, looking downwards on the slaves helps some too; less things to render below you then ahead of you.

Unfortunately, you have the choice...
a) render everything at the size of the large screen, and then resize the slave windows downwards.
b) render the slaves at their smaller size.

Option a, is what gives the accurate mouse clicks for repeater regions and general broadcasting.
It is what gives you instant pip swaps too.

Option b, is essentially what most of the other boxing options seem to do.
The PiP isn't fast, mouse isn't accurate etc.
But it plays a bit more smoothly.