When you have a wired connection it is just your computer connecting to the network via that 100Mbs or 1000Mbs (1Gbs) cable. Once you connect via Wi-Fi other factors come in to play:
In 2016 the basic router comes with a 150Mbs Wi-Fi capability, but with better units it can increase to 1750 Mbs.
The futher you are from the WiFi access point, the weaker the WiFi signal so the speed drops so you experience poor wifi
10 devices each connecting to the same 150Mbs access point each get 15Mbs, despite what speed the device is saying it's connecting at, resulting in slow wifi for each device
There are two main frequencies for Wi-Fi and you will find that most people are using the common 2.4GHz, so that other peoples Wi-Fi, in other organisations / rooms / buildings with produce a “fog” of background Wi-Fi noise which will help mask your signal and you end up a with poor wifi and slow wifi connection.