Does Wifi signal strength affect link speed?
Yes. The 802.11 Medium Access Control, which is in charge of deciding on the data rate between two WiFi devices (which use IEEE 802.11), scales the data rate between the devices depending on a lot of things. One of them is signal strength. This correlation isn't necessarily linear, but it can be. Some packets are transmitted at 1Mbps while data is typically at the max speed. When the signal is not stable, it's common for the data rate to vary quickly. For more information see 802.11 Wireless Networks: The Definitive Guide, Second Edition