Those who play on multiple accounts at once, what method do you use?

I'm trying to use an alt for the first time, so I can sell stuff without taking up time for grinding.

I downloaded Roblox through the Microsoft store, so I can run Roblox twice at the same time.

The only issue is that only the active window runs smoothly. The window in the background is incredibly laggy until I make it active.

How do y'all run multiple windows at once? I'm sure there's a better way to run this, but I have no idea where to start.