Depends on what the trigger has the unit do.
There is this many page sticky:
http://www.hiveworkshop.com/forums/f269/things-leak-35124/
Things that leak covers a lot of ground.
Things that make your computer lag is number of objects (doodads, destructibles, units, blah) on the map, or things that leak, or just having too many things happening all at once say 50 triggers firing at the same time.
Periodic events that check every second are ok if you have only one or two, if you have several then the computer starts to slow down especially if you are actually doing things like creating units, moving units, blah every second.
Objects: the simple flower model, in itself it is a simple clean model, one or two in the camera field has no impact on the speed of the computer, spam the area with a field of flowers and try the game, suddenly its running slower. the greater the camera distance, or the more objects you have in the camera the slower the computer will go - mind its 'drawing' all of those objects graphically the more polygons in a model the more computing speed it needs. WCIII is composed of low poly, medium resolution models to save graphics rendering power of your average computer system.
As for your triggers in and of themselves they will take about the same amount of processor speed/power to be completed. However it is usually best done with If/then/Else functions simply to keep your menu bar neat and tidy and make it easy for you, the designer/editor to keep track of things.