What is the real cause of lag?

Hi i was just wondering what the real cause of lag is in blender and how to stop or at least reduce it. I am mainly talking about hi poly (3000000 vertices at the least) scenes. I created a similar scene on my 10 year old laptop (intel core 2 duo, 2gb ram,ati radeon hd 3470) and it lagged but still managed to render without crashing. Now i have just moved the same scene onto my new desktop (i5 3570k, 16gb ram, and only integrated graphics) and this still had a similar amount of lag. Is it the fact i don’t have a graphics card that this is happening? Would a graphics card (gtx 660) help with horrible lags??

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Don’t know much about it, but I’m pretty sure some of the problems come from blender using opengl 1.4, and deprecated selection functions. I did try the selection speed up patch (the devs name escapes me - someone will be able to confirm) and selection speed was greatly increased. VBO’s never seem to help me too much, my advice is get the display tools addon so you can turn off double sided (on all objects at the same time), this helps me alot. Then turn off outline selected (display in N panel), that also speeds it up alot for me. Other than that theres work currently going on to improve the viewport performance.