Wi-Fi is a unidirectional (one way at a time), low-power radio connection between the modem and a device like laptop, phone or tablet.

Wireless speed vary because of the following:  

• The location of the modem in your house  
• Distance from the modem
• Physical obstructions such as walls, floors, metal appliances or furniture (ie. Fridge, filing cabinet, magnetic whiteboard, TV's, turret computers)
• Radio interference from other radio and electrical sources. For example, kitchen equipment (microwave ovens, refrigerators etc), baby monitors, garage door openers, cordless phones, florescent bulbs, hair dryers, electric motors
• If you have a Sure Signal device, place it at least 1m away from the modem
• Channel Interference from other Wi-Fi sources on same or adjacent channels including neighbours modems
• Other people using the same Wi-Fi connection, even if they are only connected and not actively using data, and what wireless standard they are on.

Also useful to know:

• Which wireless frequency is being used by your device - 2.4Ghz (further) or 5Ghz (faster)
• The number of simultaneous data streams that the modem and device can use to talk to each other. This is called MIMO and generally matches the lowest number of aerials the device or the modem has. For example: Samsung S5,S6 & iPad Air = 2 simultaneous streams, iPhone 6S = one stream
• The bandwidth of each stream when using the 2.4Ghz band. If the modem supports it, some devices can use double the bandwidth, therefore goes faster
• The Wi-Fi channel when using the 5Ghz band (use channels 34-48  or 149-165) - not speed related - but reliability and dropout related.