I could never understand why I was getting extremely laggy in mesh houses, to the point where I was unable to move. Same for when I went shopping and the store was mesh. This makes my Second Life experience not very exciting and to be honest very frustrating. I have so many stunning mesh homes, that I’m unable to use, but also so many which I really don’t have a problem with, so what is the difference? The answer: Polygon. A mesh object (clothing/houses/furniture) may only be for example 4 prims but might have a massive polygon count, which brings your viewer to a grinding halt (2-5 FPS).
So how do you know? There are two ways. but I know that Firestorm does not support the first one, or I might just not know how yet :
1. Open the build tool, then select the item. The build tool will show the triangle count. According to some a reasonable triangle count should be Footwear: less than 15.000. Pants/Shirts/Jackets: less than 25.000, if they have buttons, pockets etc. Skirts: less than 7.500
2. But who has time for that, you can also go into wireframe view and if a item looks more like a mosquito than chicken wire – it will have a high triangle count.
So if you know your PC cannot handle a high Polygon count, then buy the demo first, check in wireframe if its dense or not and then decide if you want to buy it.
Hope this was helpful.
This top is from .Mes Sucreries. for the new round of She & Him Event starting on the 5th September as well as the lips from Pink Acid.
Skin: [theSkinnery] Sasha – honey
Hair: ^;^CaTwA^;^ Mesh Gaia Hair 
Top: .Mes Sucreries. Breast Longtop – She & Him Event
Pants: erratic / briony – shorts / red
Bracelets: Izzie’s – Angular Bangles (15 colors) – She & Him Event
Make-up: Pink Acid Exotic Ethnic Lips 2 Pack – Burnt Cherry