If you are using 2 thralls(War party) then none of your buffs will transfer to your followers, so that is a waste 
Also as some have suggested, in Exile Lands there are ways to quickly level thralls from 0 to 20 in one go 
However on Siptah you are in for a very slow process, vaults used to be the way to go, especially the Spider vault, but it doesn’t give nearly the amount of xp it used to, the same goes for killing skeletons in X’Chotl, Tower and New Luxor, it is so painfully slow now 
I just returned to Siptah some weeks ago, and I am about to give up on levelling thralls, hopefully there is a faster way that I haven’t thought about, but stating that thralls taking a max of 4 hours to level is kinda… just a claim… Which thrall types? Where and what you level them with is kinda missing 
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