Switch to Sreed and/or hidden line view when weight mapping and see if that makes it faster. There are a ton of calculations going on when weight mapping and depending on how fast your system is at OpenGl it will lag.

My twin Xeon workstation, even with 3200 GPU pipelines, comes to a crawl when weight mapping high resolution meshes with the OpenGl preview turned up. (All hardware options on and decent texture resolutions) Many OpenGl calculations are basically single core operations passed between the CPU and GPU. And they end up waiting on each other when they are passing that much info back and forth. Switching to Sreed and Hidden line drastically cuts down on the information being passed back and forth.

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