I don't know how RenderMonkey is now since it has reached end of life and AMD no longer supports RenderMonkey.
If you write your own opengl program, you will get the warning message like "Large size of this xxx shader may cause slow execution". It means that the big array causes the scratch buffer using, the poor performance may be expected. We strong recommend you to using other ways like uniform array, texture buffer and so on.
If you still have problems when running your program NOT using RenderMonkey, please tell me.
Sorry for the inconvenience.