Startup screens, pause screens and bezels, eh? I think I would like to see these features be implemented at some point, especially the first two. So to answer your questions directly, Jason...
How important is it to you for the startup screen to fade into the game? Is it worth the performance sacrifice to you while playing the game?
Simply put, I don't see fade-ins for the startup screen as very imporant to me.
As asthetically pleasing as such a feature would be to see in action whenever I boot a game up, I'm perfectly fine not using it if it would hamper the performance of whatever game I'm playing. As long as the games themselves boot up properly as normal I can live without the fade-ins if I'm going for maximum performance (which is usually the overwhelming majority of the time for me), although I certainly wouldn't object to seeing this feature implemented if it can be something optional that the user can toggle on and off whenever they wish. If you've got a capable enough gaming PC, I don't see whatever performance decrease you'd get from using the feature being a serious issue if maximum performance during emulation isn't top priority to you for whatever reason.
How important is it to you to use bezels? Are they worth the performance sacrifice to you?
Much like the fade-in feature in the startup screen, the option for bezels is not very important to me personally, perhaps even less than the fade-in feature.
I typically don't use bezels for anything I play (PC games, emulation, etc..), but much like the fade-in option for the startup screen I wouldn't mind seeing bezels implemented as an optional feature that users can toggle on/off if they wish to use them. Again, maximum performance during emulation is usually what I prefer more than anything else.
What features are you most looking for with startup screens?
If possible, I'd like to see any of the following features implemented with startup screens:
The ability customize theme of the startup screens, especially if I want them to be visually cohesive with whatever themes I am currently using for Bigbox. Saving custom themes for individual games (like if I want some random image file, like fanart, for a specific game to show when I'm booting it up through Launchbox/Bigbox) if I don't want to use whatever universal startup theme I have set up would also be welcome.
The ability to view/use the game's title in Launchbox/Bigbox, overlaying whatever custom theme you have implemented for the game itself (either the universal startup screen or the custom startup screen you have set up for that specific game). Being able to modify the font for the title would be nice, if you wish to make the title during the startup look visually cohesive with whatever startup screen you're using.
The ability to view information on what emulator the game being booted up is using, if the user wishes to toggle such information on during startup.
There's not a whole lot I can currently think of that I'd like to see implemented in startup screens. Your last question, on the other hand...
What features are you most looking for with pause screens?
If possible, I'd like to see any of the following features implemented with pause screens:
The ability to view images of the games such as boxart, screenshots and fanart, with additional features suchs as zooming/panning if possible.
The ability to view information about the game itself (like the game summary) and its metadata
The ability to view game manuals/PDF files.
The ability to view cheats/hacks for the game currently being played.
The ability to view gamepad layout for all controllers currently connected to the PC, along with which controller ports/sockets each controller is currently plugged into.
The ability to configure button layouts/key bindings for currently plugged-in gamepads and keyboards/mouses
The ability to move currently plugged-in gamepads to different controller ports/sockets.
The ability to view and open up whatever emulator you have set for the game currently playing, if you wish to tweak any of it's settings during gameplay.
The ability to save/load save states.
The ability to reset games you're currently playing,
The ability to change discs of any multi-disc games you're playing
The ability to quit the game altogether and return to Launchbox/Bigbox.
The ability to integrate supported third-party applications such as MusicBox and RetroAchievements (to quote nadonate)
... And I think that's pretty much everything. If I think of anything else worth suggesting, I'll be sure to make another post on this thread. Jason, I wish you and anyone else you're working with on the Launchbox team the best of luck if you're going to give implementing any of these features a try - I'll be rooting for you!