while leader exp is imo very slow it is far better then 1.4 and befor
atm(1.5.1) you need 50 Division_hours for every exp, that is 50h for a single division fighting or 2h for 25 divisions fighting
different traits can be earned simultainiously, so attacking hills over river with pure inf gives exp for hillfighter, engineer and infantry leader
this amount is independent of the lvl of the leader so a lvl 9 general gains traits as fast as a lvl 1, in 1.4 the division;hours per exp scaled with the lvl so a lvl 2 leader needed twice the time to gain the same hillfighter trait
FM gain 1 exp per 100 Division_hours so with 5 armies that is 2.5 times the exp each general gets, assuming all divisions fighting all the time
that being said, 50h per exp with most traits needing 500-1000 exp gives about 25k-50k division_hours for a single trait or ~1k-2k division_hours with a full army, that is more than 40 days of fighting with no break at all for ALL 24 divisions
gaining a lvl has the same exp per division_hours but is being gained all the time with no restrictions
you need 200 exp to go to lvl2 and it doubles with every lvl so lv 5->6 is 3.2k exp or 160k division_hours => 6667 hours with 24 divisions or 277.78 days with full army of 24 divisions
but the best example is naval invaderm which takey "only" 100 exp
this translastes to 5k division_hours or 1250h of full 4 divions being in naval_invasion_battles, that is more than 52 days of constant combat if you only use a single invasion at a time
for a lvl 9 FM to reach lvl 10 you would need 102400 exp or >5m division_hours, from lvl 1 you would need 204600 exp or >10m division_hours
that is >83k and >166k hours of permanent fighting with 5 armies of 24 divisions = 120 divisions or 200k/400k hours for the general with 24 divisions
for the FM that is almost 20 YEARS of non stop fighting to go 1->10, so ever so slightly above the games timeframe of 12 years, or "only" almost 10 YEARS for 9->10,