I had low FPS (20-30 with CPU at 99% utilisation) and couldn't get full screen working (wasn't filling the screen on a 4k TV connected to laptop). Here's what I did to fix it for my setup.

Using AMD CPU + Nvidia 3070 laptop GPU.

FULLSCREEN
Right click the game in steam > Properties >Installed files >Browse > right click dis1_st.exe > properties > compatibility > change high DPI settings > tick override high DPI scaling behavior and change dropdown to application. Now I can run the game in 'Fullscreen A' properly.

FPS
Add the game to Nvidia control panel
Manage 3D settings > Program settings > Add > find it on the list> scroll down near the bottom of the settings to Threaded Optimisation and set it to OFF.
FPS went from about 20-30 to locked 60 by disabling threaded optimisation for it.

This also worked for Disgaea 1 btw. Hope this helps.