I changed some of the game options such as demense and vassel limit, and turned off regency etc. Now my game runs more slowly.
Is there any info on what is the optimal game setting choice for the fastest game?
Not yet. Feel free to do some Science!
Are the Bayaji Jungle Ambushers the only retinue available to Shadowrealm? If not then what are the requirements to unlock the others?
Far as I know, they have nothing that sets them apart from regular-ass Bayaji. If you get an Imperial Mahtmasi, that might be different.
How to learn magic ? Every time, no matter how much I invest, I never learn any magic. It always fails after some time or I die because of an experiment gone wrong.
You have to be ridiculously lucky or insanely smart to begin with (read: High learning stat , at least close to 20), as investing money gives you more negative traits like slothful, while experimenting tends to blow up in your face while offering advantageous traits.