Why fps reduces when i draw 1000 cubes at same position?

Hi everyone,
I have been working in webgl for quite some time. I am now trying to optimize the performance of my webgl application. When I draw 1000 cubes at different locations(transformed pos), I get around(56~58) fps. But, if I draw them at same location fps drops to 11~13. I thought it might be because of depth testing. I disabled depth test but no improvement.