I don't think it should affect it that much. Lower signal strength just means you are meeting a certain level, similar to your cell phone. Whether you have 5 bars or 2 bars, it still works, right?. After all, with digital you either have the packet or you don't. The only downside to a lower signal meter is possible it being more suseptible to interference and possibly might lose more packets in transit, which would cause the need for retransmission, so that might be what makes it slower.
The connection speed varies according to signal strength. For my dlink card, full bars gives me 52.5mbps, and goes down as far as 5mbps at its weakest.
So depending on signal strength you can notice the difference just web browsing.
Tell you what...It is advised not to have microwave oven or cell phone put close to the wireless router. this is because the router should be operating at 2.4Ghz frequency so the signal might be disrupted by the use of the microwave oven or cell phone.
internet speed will always be slower over wireless no matter how close you are comparing it to plugging it into the router (i dont know why, but it just is. The internet speed itself shouldnt fluctuate much unless you have cable or share a line, but if your connection speed on wifi exceeds your internet connection then it shouldnt be too bad