Same problem with my Macbook Air Mid 2012. In the last three days I did everything I found written here with no luck, including fresh copy of High Sierra. Today I thought to reset the router, Aiport Extreme. First exported a baseconfig file from Aiport Utility (File > Export Configuration File) and after that HARD RESET (disconnect the router and while holding a paper clip in the reset button power on and hold the clip/reset pushed for 6 seconds until the led flash amber rapidly). After that open the Airport Utility, select the base station/router, click Edit, Choose File > Import Configuration File, locate the previously exported file, click open and update the settings on base station.
After that everything returned to normal, my 300 Mbps connection is back!
Almost forgot to tell, before the hard reset, 4 Ipad's, 3 Iphone's and an Imac (Sierra) in my house worked at normal wireless speed the only one with problems was the Macbook Air with High Sierra (although it was connected all the time at the 5Ghz channel).