Is it a common occurence for internet speed to drop over a Wi-Fi Router?

Sorry for the noob question. On my network switch I have the following connected - Internet modem, Router (TP Link Archer C7), and PC. Speed tests on my PC get me 50 up and 50 down, which is the correct speed. Speed tests on my iPhone via the router get me about 39-40-ish up and down. Is the drop in speed a common occurence via Wi-Fi?

Yes, and the more things using it at the same time the more likely it is to slow down. Also depending on which ISP you have, other people using that one can cause all of them to slow down. I know that our cable company here has a lot of problems with that as the more people that log on the slower they all get.

Yes…

Added after the "Best Answer" vote was made:

Ignoring the load on the ISP, which is known as contention ratio, whereby the ISP effectively oversells their connection to the Internet backbone. As an example, although not real figures, an ISP may have a 1 Gbps connection to the backbone, and may sell contract one hundred 50 Mbps connections. For much of the day when someone does a speed test they will get 50 Mbps; however, 100 x 50 Mbps = 5 Gbps. This means if all their 100 customers try to get their maximum download, then they will only get 10 Mbps each rather than the 50 Mbps they are paying for. Anyway that is just an explanation of one part of the best answer.

Ethernet is generally faster than Wi-Fi, although there are examples of Wi-Fi being faster. Whenever you are testing your ISP connection is is essential that there's nothing else using the ISP connection at the same time. At 50 Mbps, many Wi-Fi networks and Ethernet are capable of speeds significantly faster than this. I pay for 52 Mbps, and I normally get 50 to 51.5 Mbps either over Ethernet or Wi-Fi as both these interfaces are significantly faster than my ISP connection.

There are several factors that can affect speed particularly when connected to the router by Wi-Fi. The distance of the computer from the router affects speed. The greater the separation the slower the Wi-Fi interface. The other thing that affects speed tests is the processing power of the device. It is quite likely that your iPhone has less processing power than the PC, and the phone is not capable of processing the speed test properly.

No…

But its a Common Occurrence for WAN speeds to fluctuate.

Wi-Fi can handle the Bandwidth of Internet connection… Just lacks in the reliability of Wired which compromises the speed of Wi-Fi connections.

Speed (latency) and Bandwidth (size) are two different things which both determines how solid your connections is.