A lot of calculations.
If you have a high exp server, and weapons dealing much damage, the computer have to "calculate" everything. In small terms its fine. But with messy spells, high hp/mana on players, monstesrs dealing much damage, you dealing much damage, healing much, messy events. All together this gives the CPU alot of strain.
When any OT reach 150+ players online, proper scripts starts to become necessarily.
For an example of you have shop system and global events that runs every second, it will run every second on each player. If it took 3 milli seconds to run this globalevent, with 200 players it would take 600 milli seconds. (Which is over half a second lag) every time the global script runs.