- draw above 10,000 polygons in vertex buffer should be faster
- in theory, vertex shader and geometry shader have the similar performance ( emit vertex per clock), so if you need handle same vertices, the performance should be similar, but actually additional pipeline will hurt the performance.
- geometry shader output is write to video memory, so use geometry shader can save memory bandwidth.
- you can use timer query to measure the performance. please reference http://www.opengl.org/registry/specs/ARB/timer_query.txt for detail.