So, I am assuming this would work much like it would on an android phone. You can find a launcher in the app store, or find a 3rd party apk to install (if you trust it).
My questions is whether or not this has any effect on the responsiveness of the TV.
I have a Hisense 55H8F which has gotten incredibly slow despite the setting changes I make. Would a new launcher change anything, or would it take up some of the already sparse RAM/CPU power?
It works simular to the phone style. You can install one from the Play Store or sideload it.
But, most devices don't have a way to set a default home like phones do. So you either need to remap buttons, use an Android TV launcher manager (of you trust it), uae ADB (which might have unintended consequences), and some apps have built in support for trying be default (Sideload Channel Launcher 2/3).
But, its still running your device, so if your upset with the performance of the device itself, it's unlikely just a new launcher would fix it, but work trying.
FLauncher us free and FOSS, so it's a good way to teat it out.
Otherwise, get an external device with faster hardware. That's how most people get around underpowered TV hardware or eventual lack of support.