What is 802.11ac WiFi?

802.11ac is currently the most accessible WiFi standard which provides high wireless speeds on the 5GHz band. The standard is often referred to simply as "Wireless ac" or "WiFi ac", much like its predecessors 802.11a/b/g/n are referred to as "Wireless a/b/g/n" or "WiFi a/b/g/n".

Despite being the most accessible WiFi standard 802.11ac isn't the fastest. Introduced in 2019, 802.11ax, also known as WiFi 6, is currently the fastest WiFi standard.

Speed

The 802.11ac standard is capable of fast speeds generally between 100 to 400 Mbps depending on your router, the client device, and the environment.

Most of mobile technology built after 2015 uses 802.11ac. With the majority of newer mobile phones, built after 2019 being capable of 802.11ax as well as 802.11ac.

Keep in mind 802.11ac, despite being widely available, only came about in 2013 and wasn't widely accessible until 2015. If you are using an older router that was manufactured before 2015 you are limiting yourself to the older and slower WiFi standards of its time.

Standard	2.4G or 5G	Theoretical Max Speed	Typical Real-world Top Speed	Introduced
802.11a	5 GHz	54 Mbps	approx 20 Mbps	1999
802.11b	2.4 GHz	11 Mbps	approx 6 Mbps	1999
802.11g	2.4 GHz	54 Mbps	approx 20 Mbps	2003
802.11n	2.4 GHz	150 Mbps	approx 70 Mbps	2007
802.11n	5 GHz	150 Mbps*	approx 90 Mbps	2007
802.11ac	5 GHz	866 Mbps*	approx 225 Mbps	2013
802.11ax	5 GHz	3.5 Gbps	approx 500 Mbps	2019

* indicates "per stream."

Achieving even faster speeds:

On the 5 GHz band, the wireless N and AC standards support a technology called MIMO, meaning "multiple-input, multiple-output". Modern routers have multiple 5G antennas and can send data to multiple devices at once (to devices A, B, and C all at once... rather than A, then B, then C like older routers).

MIMO also allows the combining of multiple 5G antennas together when the device (computer, phone, etc) also has more than one 5G antenna in it. This is how certain devices can surpass the approximate real-world maximum of a single 802.11ac link.

Example:

If a device has two 5G antennas (sometimes referred to as a "2x2" antenna), it can bond the 2 together. The effect is best achieved when you have line-of-sight to the router and while not a perfect doubling of speed, it can be a drastic improvement.

The Telco Router supports MIMO, and has 7 antennas (three on the 2.4G band, four on the 5G band) -- and most modern 3rd party routers do as well. 

Keep in mind most client devices do not have more than a single 5 GHz antenna and will not benefit from the benefits of MIMO technology, but some higher-end devices may.