The speed is always going to be slower at weaker signals. To get the maximum speed that is currently available with your current setup, you need a stronger signal.
If my wifi is being used by 10 computers at once and I have a crappy signal on top of that, the speed is going to be slower than if I had a strong signal (even if 10 people are still using it). Get it? It's not rocket science.
Now you've said it correct. What you were saying before was false. Having a stronger signal does not mean faster speeds alone. I'm sure a lot of people get much faster data speeds then I do with less of a signal. I've already stated that I'm generally always full bars wherever I go, yet my data speeds aren't very fast.