Q's on performance, theorycrafting and friends

1. Not that I can see. For battery drain, there is an option to turn off push notifications for AP and BP recovery, but I can't see any option to reduce the game's visual and audio quality, which is what you'd normally do to make it run more smoothly.

2. I'm pretty sure most of the fgo channels do that? The most prominent are probably G&A Reviews and Shotgun Shogun. I'm not a fan of these channels in general since all they really do is regurgitate what you'd find on literally any google search (fgo wiki, etc), nothing you can't find out faster yourself, but if you're interested in seeing this information in an audiovisual format, those are what I'd recommend. Personally I find Voice of Saint Quartz to be more worth the watch despite the fact that he doesn't have nearly as frequent an upload schedule and he tends not to comment as much on the state of NA as it goes along. The guy running the channel has confirmed that he will make one of those "servants to save for in 2019" vids though so I'd recommend that at least.

3. So far on NA the limit is 70 because the master level is capped at 130. In JP it is capped at 150, so theirs is 90 (as of latest singularity release). Friend limit increases as you level up, 1 per level.

1. Indeed. FGO in my iphone 5c always crash, but ipad 2017 worked excellent. Even as back platform. But generally wont change too much
2. Well, I am not sure but it is not hard anyway. Sorry
3. Nope. it is JP with lv140-150 master level.

I think speed depends on the specs of the device itself. I have played on 3 different tablets. One was a cheap and low spec affair... The entire app usually crashed 3-5 times a day, usually during a np animation. Then I switched to a Samsung Tab A and my alternate account is on an iPad mini. I've noticed when farming in both accounts simultaneously, I can get through the same battles faster on the iPad since it generally spends half of the time on the loading phases, but will still occasionally crash on np animation, but the Samsung never crashes. Not sure what specs are specifically better, or if it is Android vs iOS.

2. FGOTips, Voice of Saint Quartz, and Chaldea Gurus provide the most unique content and are my recommendations.
Out of the ones that constantly post guides, TouhouSniper98, G&A Reviews, and Fino Play are good with TouhouSniper being one of the more prominent whales in the game.
For Servant in-game analysis or spotlights, besides the ones above (With Voice providing pretty detailed analysis) Honako Green and Lily Aquina show off the capabilities of servants.