Having to wait 3-5 seconds for a response is not typical, regardless if you are using DDR RAM or the older SDRAM.
It would appear that your PC is busy doing other tasks. I would try to bring up Task Manager, and order your processes by CPU time. This should tell you what processes are currently utilizing the CPU. You can also use Task Manager to check the memory stats, and see if you have utilized all your memory, and the PC is having to swap memory space to disk. This would definatly slow your machine down.
An easy way to sort out your problem is to reboot your pc. Start all your nomal program, one at a time while checking your system response. If you have one applicaiton that causes a slowdown regardless if you load it first, then you have an application that is running at a high priority and taking controll of you CPU.