This is because of the way it works. Your hard drive is nowhere near as fast as RAM is. That's why we use RAM in the first place, because it is so fast. On to RAM Doubler. I never understood why users defend such products and say that they do not affect your Mac's performance.
Stability also becomes a big issue.
I have talked to dozens and dozens of people about their experiences and most have come to the same conclusion that I have. Although there are exceptions, a whopping majority had crashes related directly to RAM Doubler. Its RAM compression and virtual memory DO affect your Mac, especially when you execute demanding tasks. Those who use G3 computers can fall easily for memory solutions. Those beasts are so fast that a performace difference looks slight.