Niri demo video actually looks kinda cool, could be nice to use on a laptop when there's no access to multiple external monitors, so that you could just pop a log/tool/whatever window to the side without fulling swapping workspaces xmonad/i3/hyprland/etc style.
But with 2+ screens available I'd think at least for me the usefulness would diminish, even if you'd have per monitor scrolling
Spring Boot startup time is indeed a problem, especially when scaling horizontally on low cpu nodes.
If your environment allows for burst cpu usage until ready to accept traffic, you can start up really fast as spring does so much reflection magic during startup that can't be done during compilation "trivially". You can include hints for runtime configuration from a build, but it doesn't do much to help in really low cpu envs.
Then you can of course just do native images, but you lose some of the spring "magic", and might be annoying to refactor towards.
But with 2+ screens available I'd think at least for me the usefulness would diminish, even if you'd have per monitor scrolling