The other day, I was at a coffee shop where the Wi-Fi seemed slow. I didn't run actual speed tests, as that would have just added to the network load. Instead, I fired up the excellent WifiInfoView program from Nir Sofer.
Windows, like many operating systems, provides a pathetic amount of Wi-Fi information. Without sufficient technical data, we are left to guess at the root cause of slow Wi-Fi. WifiInfoView is the motherload of techie information about your Wi-Fi environment. I ran the program just to check on the signal strength, but learned much more.
The coffee shop had two Wi-Fi networks, one on each frequency band (2.4GHz and 5GHz). I had connected to the 5GHz band. If you are close enough to a router to get a strong signal, the 5GHz band should be faster. If the 5GHz signal strength is weak, the 2.4GHz band should offer a stronger signal as the lower frequency allows it to better pass through obstructions.