I really think that you are approaching this from wrong angle. Focus on your problem statement. Which if I read it correctly is that there is too much lag cause by monster pathing when they are not removed.
I see nothing in there that say we need to reduce speed. Sure it might help if that means more monsters killed, but at the expense of really annoying a lot of players (a lot player that like to pay for things like xp pots).
What do the players want?
1) We don't want to get less xp/min
2) We don't want to chase mobs all over the place simply to get kills (melee toons and ranged mobs on platforms... annoying)
3) we don't want to have to play a nuker caster simply because we need to one shot rooms and hit things at range while hoping to keep kills up while doing leveling for lives (Granted we may play then anyway, but don't force us)
Now, I realize the dev's also have objectives
1) not making this so easy that people get bored and leave
2) controlling lag, so people don't get annoyed and leave
3) any other things to keep revenue up, etc. (btw, I have issue with this concept, it's a business).
With those in mind, we can probably think of some options
1) Reduce the # of monsters but make them more powerful. (might also address overpowered casters). Few monsters, less pathing, but still the challenge
2) base conquest on mobs agro'd, not possible mobs in the instance. Doesn't slow us down, but then that shouldn't be your objective anyway.
3) Lower the xp needed in general for levels, reaper points, etc. Have your changes, but ensure our grinding isn't made worse
4) Add the ability to one shot rooms to Melee and Ranged. Yeah, okay, I really don't like this idea, but it's like #3 but addresses non-casters will suffer the most from your changes
5) some combination of the above. I like #1 and #2. Make fewer, but more powerful mobs and force us to take them out.
We've had a lot of great feedback in this thread, but one thing that we haven't seen a lot of feedback on is suggestions to get players back to the XP/minute and general questing speed that was taking place prior to Update 59. We have seen a very significant increase in general speed of play since our recent lag reduction work, and as players have noticed, it's causing issues regarding game performance. Some of the goal here is to get players back to the pace they were prior to those recent changes. So, just to ask: How would you reduce player speed as it were to pre-Update 59 levels?