I recently purchased an iMac, and upgraded the memory to 8GB, however, my iMac has been really laggy these days, so today, I checked the memory usage. It turned out that Dock took up 3.9G of memory, how could that be?!
It's not like if you use only 4GB today, you can use 12GB tomorrow. And the memory being in use has nothing to do with why your computer is laggy. If a computer is laggy, you speed it up by making it use more memory, not less. If your computer were using less memory, it would be slower. Otherwise, we'd remove memory from our computers to make them faster.