'Turn 20' is when aliens spot your troops via mind bullets and come out of their UFO to kick ass be killed like lemmings. The setting that controls that is 'cheatTurn'.
I don't think it's possible to make vanilla AI appreciably more aggressive. You could increase the 'aggression' and 'intelligence' values for individual units, I guess. I haven't noticed that making too much of a difference, though.
Well, melee enemies can actually be made dangerous by changing them to leeroys, like Zombies already are. Personally, I quite like leeroying cryptids or Chryssalids, because now they're dangerous and the mission is much less likely to turn into a bughunt. But that's only a small subset of all enemies. There's even a submod of mine for that, though it's not very up to date.
Big maps and showing more layers or more terrain have always been a performance headache, BAI or no BAI. XCF changing the vision range to 40 doesn't help there, either. BAI bugs should go into the relevant thread, though, since only Xilmi can resolve them properly.