The likely result is a Linux kernel update. One of the updates from 3.18 to 4.1 to 4.4 to 4.9 to 4.14.
Example: routing performance deteriorated in a major way between AA12.09 and BB14.07 as the routing cache was remove from Linux kernel in kernel 3.6, so the performance of AA12.09 with kernel 3.3 and of BB14.07 with 3.10 differed quite much. Same has happened with other major kernel version jumps, so finding the Openwrt commit may not help that much, if the commit is a major kernel version upgrade (as I guess is very likely)
(You might read a longer explanation in R7800 discussion in Possible cause of R7800 latency issues - #55 by hnyman )