Well, consider that every NPC already has a radius check when they check for people entering aggro range. So, this system would essentially only be extending that radius, which means it probably wouldn't cause more server load in that aspect.
Then, consider that NPCs that might have running scripts, pathing, spell casting, and other checks running continuously would now only be running them when players are within range.
Also, I can repop an entire zone at once with only a blip of a performance hit. If only 2 or 3 spawns are popping per second while players are running around, I don't think that would cause any noticeable impact on performance. Even if they were moving fast enough to spawn 20 at once, it would only happen while they are running around, not while they are fighting, unless maybe if they are kiting. And, raid fights are my main concern for reducing lag. Raids and AE groups are the only real things that I ever see cause performance hits on my server.
And for players moving at really high speeds, normally that would be in open zones outdoors. In that case, you could either leave this system off for all NPCs in the zone, or set them to have a really large radius to ensure that mobs are popping on top of people. I don't think anyone moves faster than 600 feet per second without warping hehe.
Another bonus to this feature is that admins could setup zones that MQ maps are useless for. Players wouldn't be able to see which spawns are up or down on the map unless they actually entered the nearby area. I imagine that could be useful in many situations for those anti-MQ admins
The nice thing about this idea is that it would be a fully optional setting. You could leave important NPCs like Named and other scripted NPCs up all of the time by setting them to not use the system. This system is mainly to get rid of unneeded trash spawns.
I do think that the coding for this might be pretty easy to do. I am going to look into the aggroradius code and see if there is a way to possibly combine that with the spawn2 table to make the server watch for players to enter a set radius around each spawn point.
Yeah, not having an NPC there to base the radius check is probably my main concern, but I still think it should be possible. And the potential for very noticeable performance increases should be worth at least looking into this further.