Reason of why Khuzait is 90% strongest faction is this :
Khuzait parties have 32% cavalry ratio while others have 12% average. I totally removed all speed bonus effect of cavalry units (60%) and footman with horse at inventory (30%) too see what will happen and result is this at 11th year of game (you probably first see something like this which Khuzait left only 4 towns without player interaction at late game (10+ years)) :
This is not intended. This only happens because of wrong design decisions (deciding something without thinking side effects). There is 60% speed bonus for cavalry units currently (means if your all party is cavalry you get 60% speed bonus if half is cavalry you get 30% and so on...). Lets say average party speed is 10 for 100 men all footmen party. Then average Khuzait party consist of 32% cavalry troops their speed become 10 + 10 x 0.6 x 0.32 = 11.92 while others are 10 + 10 x 0.6 x 0.12 = 10.72; so mostly Khuzait parties can run away from stronger parties and can catch weaker parties. Always decider is Khuzait party in party meetings. They can catch or run away. So they usullay enter battles which they can win they collect more loot they lose less battles they save their troops from stronger enemies they have more money they have stronger garrisons because their economy is good even they collect recruits faster (even this is minor). When they start a siege if defender army come they can run away. They can defend their settlements easier they can go sieged town faster (minor effect too). All are positive.
What can be done :
1-We should increase cavalry ratio at other factions. I am not saying all should same but if Khuzaits have 32% cav ratio others should have 15-20% not 12%.
2-We should lower 60% mounted units speed bonus to 40% or 50%. 60% is so much and so OP.
3-We can give several disadvantages to Khuzaits like less recruits or something else.
Even we do these still problem will be there (according to how harsh 3 is) but it will be less problem at least. I added first 2 solutions to 1.5.7 and average snowball score dropped to 50s from 60s.
I am ok with unbalanced design if Khuzaits are a bit better (fe 40% of time they have dominance at late game) but currently they are not a bit better than others there is a major difference (90% they have dominance at late game). After snowballing fixes at 1.5.6 they can get at most 50-60 fief points at 20th year of game in past this was about 70-80 fief points (towns 1, castles 2). In 1.5.7 it will be 40-50. (All world is 173)