Painfully slow WiFi after Sierra update on Mac Mini
Apple has a post about reccomended wifi settings for your router, as they do not believe that it is their software being glitchy, but the interference between bands (I've had the issue as well. See https://support.apple.com/en-ca/HT202068 or see below)




  2.4 GHz channel width
  
  
    Channel width controls how large of a ”pipe” is available to transfer data. However, larger channels are more subject to interference and more likely to interfere with other devices. A 40 MHz channel is sometimes called a wide channel, and a 20 MHz channel is a narrow channel.
    
    
      Set to: 20 MHz
    
    
    Use 20 MHz channels in the 2.4 GHz band. Using 40 MHz channels in the 2.4 GHz band can cause performance and reliability issues with your network, especially in the presence of other Wi-Fi networks and other 2.4 GHz devices. A 40 MHz channel might also cause interference and issues with other devices that use this band, such as Bluetooth devices, cordless phones, and neighboring Wi-Fi networks. Routers that don't support 40 MHz channels in the 2.4 GHz band do support 20 MHz channels.
  
  
  5 GHz channel width
  
  
    Channel width controls how large of a "pipe" is available to transfer data. Larger channels are more susceptible to interference, and more likely to interfere with other devices. Interference is less of an issue in the 5 GHz band than in the 2.4 GHz band. A 40 MHz channel is sometimes called a wide channel, and a 20 MHz channel is a narrow channel.
    
    
      Set to:
      For 802.11n access points, set the 5GHz band to 20 MHz and 40 MHz.
      For 802.11ac access points, set the 5GHz band to 20 MHz, 40 MHz, and 80 MHz.
    
    
    For best performance and reliability, enable support for all channel widths. This allows devices to use the largest width they support, which results in optimal performance and compatibility. Not all client devices support 40 MHz channels, so don't enable 40 MHz-only mode. Devices that support only 20 MHz channels cant connect to a Wi-Fi router in 40 MHz-only mode. Similarly, don't enable 80 MHz-only mode, or only clients capable of 802.11ac will be able to connect. Routers that don't support 40 MHz or 80 MHz channels do support 20 MHz channels.
  
  
  WMM
  
  
    WMM (Wi-Fi Multimedia) prioritizes network traffic according to four access categories: voice, video, best effort, and background.
    
    
      Set to: Enabled
    
    
    All 802.11n and 802.11ac access points should have WMM enabled in their default configuration. Disabling WMM can cause issues for the entire network, not just Apple products on the network.
  
  
  MAC address authentication or filtering
  
  
    Restricts access to a Wi-Fi router to devices with specific MAC (Media Access Control) addresses.
    
    
      Set to: Disabled
    
    
    When enabled, this feature allows a user to configure a list of MAC addresses for the Wi-Fi router, and restrict access to devices with addresses that are on the list. Devices with MAC addresses not on the list will fail to associate with the Wi-Fi network. MAC addresses can be changed easily, so don't rely on them to prevent unauthorized access to the network.
    iOS 8 and later uses a randomized MAC address when running Wi-Fi scans. The scans are conducted when a device isn't associated with a Wi-Fi network and its processor is asleep. A device’s processor goes to sleep shortly after the screen is turned off. Wi-Fi scans are run to determine if a user can connect to a preferred Wi-Fi network. Enhanced Wi-Fi scans are run when a device uses Location Services for apps that use geofencing, such as location-based reminders, that determine if the device is near a specific location.