I haven't really experienced lag in retail, on my decent computer - the only noticeable change between a build of, say, 5 bricks, and that of 128 thousand bricks, is a longer spawn time as the bricks are loaded.
Also, the computer I am on at the moment has a 32mb GFX card, and can really only run Blockland with graphics turned down to absolute minimum, and even then it is prone to crashing. It's time for you to start saving up and purchase a new card/computer.