Had an issue with my keyboard where it typed the entire top row + Enter every time I hit p.
On topic: I’m using the Honor View 20, which has an 19:9 aspect ratio thanks to having a hole punch selfie cam instead of a notch. In most non native fullscreen apps, it defaults to 18:9, leaving a bar the size of the hole punch at the top. FGO then additionally adds the blue borders you mentioned to keep the game at 16:9, making it look like this:
If your phone doesn’t have a notch or hole punch, you will simply have the blue borders, not the black one on the left. It does look a bit dumb, but I’ve honestly stopped noticing it after a while. The image doesn’t feel squished or anything since you barely lose any effective screen space. I do hope FGO adds support for higher aspect ratios at some point since every other game I’ve played manages this just fine with zero issues, but it’s not a big deal either way.
As for performance: the V20 has a Kirin 980 processor, which is about the same as a Snapdragon 845, which you can find in most high end phones from last year (this year the 855 is out which is slightly better). The game literally never lags, not even on switching servants on death or with the plugsuit, and load times are about 20 seconds at most to get into a battle. So this processor is already way overkill, and the phone cost me 500 bucks when I bought it 4 months ago. You definitely don’t need anything expensive to play FGO just fine.