Ok firstly the game will probably hang for a few frames while the unit assets and data are loaded the first time. If all 4 such units were created at once and each has many abilities with many levels this might last 5-10 seconds of the game being unresponsive, however usually for standard units it is only a couple of frames (1 second for all 4). Avoid this by placing the units on the map somewhere so they are loaded during map initialization (note it will make load times longer).
Secondly the trigger only fires once every 15 minutes and can leak at most 1 location each. This gives you a possible leak amount of 4 locations from all those triggers! This should make practically no difference to game performance at all since you need a large number of leaks (like 4,000 or 40,000) for there to be performance problems. As such I say your bad performance is coming from somewhere else and not from these triggers. That is if your bad performance is persistent and not once off as mentioned above.
Thirdly, you do mean "bad performance" or "low frame rate" right? And not lag? Since lag mostly is the result of excessive network traffic resulting in very large TCP window size. Where as bad performance is caused by your system running out of free resources to meet the game's real time requirements, lag is caused by filling up network buffers. An example of bad performance would be creating 1,000 air units at the same point and letting them displace each other. An example of lag would be playing a multiplayer session involving the internet on a standard ADSL connection while running Peer 2 Peer software with no bandwidth limit in place.
As for your trigger it self... You are aware that to have a 50% chance (yes only a 50% chance) to see these rare units you need to play for 5.7 hours? That is by no means a reasonable expectation for someone playing a WC3 map. You have a higher chance of finding a Furnace or Witching Hour playing Diablo III for the same amount of time I recon, and both those items are notorious for being virtually impossible to find.
A session length of 1 hour is reasonable for a WC3 map, anything longer and it is hard for most people to have such length of continuous free time. As such you are looking at 4 rolls per session. A good rate might be 50% chance per session (I personally would go higher but you want it to be rare). So I would say the probability should be around 0.84 of it not spawning or a 16% chance to spawn every 15 minutes.
RareTroll
Events
Time - Every 900.00 seconds of game time
Conditions
Actions
Set ChanceRareTroll = (Random percentage)
Wait 0.01 seconds
If (All Conditions are True) then do (Then Actions) else do (Else Actions)
Trigger - Turn off (This trigger)
Unit - Create 1 |cffc0c0c0Tazdingo|r (Rare) for Neutral Hostile at (Center of RareTroll <gen>) facing 300.00 degrees
