Then I got 60fps in Steam, but no image. After disabling the Nvidia overlay in the Gforce settings I had an image again but heavy framedrops. After setting Bandwidth limitation in Steamlink itself to "Auto" these were gone as well. Now Rayman is running with 60pfs, what results in no speed change of the game. Why the game reacts so badly on not having 60fps I did not found out.
I seem to remember you can limit the pixel clock instead, but I haven't tried it.
Yes, it's possible, though this will disable the 4k video modes completely.
Also, given the performance issues with 4K, wouldn't it be better to have RetroPie be limited to 1080p max out of the box, with an easy way to enable 4K if the user wishes?
This is exactly what the hdmi_mode/hdmi_group settings do - set 1080p video mode by default, with the ability to choose 4k via runcommand (though performance will not be great). Kodi/Pixel desktop would still be able to use those modes for 4k HEVC movies/normal desktop.
Unfortunately there isn't a way to configure the video so that 'if 4k is available, use 1080p by default', so instead of forcing 1080p by default for all configurations, it's something that the user must configure after installation.
@njsilvestri21 what is the make and model of your tv you are using? Some 4k TV's only support 60hz on one particular HDMI port and some off brands don't support 4k60 at all. But just an fyi, RetroPie performs much better at 1080p.
if it's screensaver / screenblanking shouldn't it happen when other screens are attached as well ? the settings about it do not change when switching screens so in theory he should have the issue on every screen attached then