what i've found over the years is that most players I've played with get slower the more options they have. The min maxers and detail oriented ones seem to be the worst. I'm sure a lot of the blame is on DM's me included that let them take so much time but also if a lot of high level spells are being cast then things tend to get hinky, adjudications have to be made and sometimes that slows it down as well.IDK what we were doing wrong, but our average high level combat took about as much time as a low level combat. Occasionally, it might take 45-60 min to finish the entire combat if we are unprepared or getting our butts kicked and need to regroup and refocus. Thats like 1 out of 6 combats though.
It can be sped up but the players have to accept that they have to make thier decisions quickly and deal with the fact that they may make suboptimal decisions in the heat of battle. I also find MMO players current and former tend to be the worst being used to planning perfection in thier play they have a hard time not being able to do that at the table.