I've read through a similar sounding problem here: My MacBook pro Wi-Fi is slow, other devices are OK.
However, my MCS index, Transmit Rate(Tx), RSSI and Noise seem to be relatively acceptable. See screenshot here:
This all seems ok from what I've read, however, the Wi-Fi still runs very slowly on my Mac. When I run an internet speed test my download speed is consistently < 1Mbps when it's above 20 on all other devices.
Best Answer
After speaking to a friend I established that this problem is potentially due to the macOS Sierra update. I followed the first step in this link - removing existing Wi-Fi preferences. Which seems to have worked for now.