Hi,
first of all I don’t think displaying the Browser on your openhabian installation will be the solution to your problem. I would keep the pi with openhabian a dedicated machine.
I have also tried to display the MainUI on my rpi4 4GB with 10’ Sunfounder screen and the performance was horrible. The whole thing just isn’t made for heavy gpu rendering and the hardware acceleration is not always used.
Thankfully, a user in this forum pointed me at an Android distribution for the raspberry: LineageOS 17.1 (Android 10) for Raspberry Pi 4
With this, you will be able to have a somewhat enjoyable UI experience for your wall mounted device. It’s not butter, but many times better than any raspian distro with chromium.
Don’t forget to install the Google apps on this thing so you can download chrome frome the play store. You should find some documentation under the link I posted.
All the best,
Bob
P.s.: Here’s the forum link with more instructions: Page rendering speed on Raspberry pi 4 (as a client) - #3 by rpwong