Well I work in IT sector server maintenance so I know what means 10.000 users online and 100.000 users working at the same time. Trust me its much more server load.
When y play 115 its casual but on 120+ y kill enemies a lot slower so y exchange a lot more numbers with server until one mob die /115 faster, 120 slower etc/.
It make sense... All over Europe we have lags since season start, sometimes y have message "waiting to log in game" and sometimes y need to change server to have decent ping for normal play /Im on 100mbit fiber btw/.
but hey if y didnt play 126+ y wouldnt know ;-)
i do too.
Did you ever evaluate a bit what kind of sources blizzard does use? Its a small difference between yoursmaller setup and the blizzard infrastructre systems. Here is an older Link regarding the used sources through world of warcraft. http://www.datacenterknowledge.com/archives/2009/11/25/wows-back-end-10-data-centers-75000-cores
The Batle.net is operated through telia in nowadays.Diablo is running on the same shared and on demand infrastructure as hots, overwatch and sc2. Only WOw has "dedicated" CLoud ressources except the instancing server which are booked on demand.
You are right with the serverload, but i think the lags are caused more through diablo3's spagetthi code than through online users.
Sorry for the OT, but technical things do catch me always a litle bit.