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The Render Weight of your avatar, is a numeric representation of the amount of work your computer and everyone else's, has to do to render you correctly. If you are at a sales event or club for example and there are a lot of other avatars around, your computer will need to render them, and if they all have a high render weight then glitches (LAG) can occur. All mesh attachments have a render weight. Bearing this in mind I try and make all !APHORISM! Items as efficiently as possible, I believe user experience is important, not just the finished look of any item. I am not an expert, just sharing some info and trying to improve my releases as I can.
You can see you current render weight by going to Top Menu > Advanced (press Ctrl-Alt-D if you don't have Advanced option) > Performance Tools > Show Render Weight For Avatars.
Here are some links to articles/blog posts/and a video about the problems with high render weights, LL's plans to bring awareness to the issue and some ways of lowering render weight for anyone interested.
I was recently interviewed for a podcast by Indie Teepee and briefly discussed the issues around render weights - listen here.