I have been having this issue with two of my 5 computers, both of them the newest ones [HP 15-ba000 series] with Intel Dual Band Wireless AC 3165 cards. One is a bit less cranky than the other, but both almost always end up connecting to a smartphone being used as my mobile hotspot at glacially slow speed. The only trick I've found to get around this, and it is not consistently successful, is to disable and re-enable the device.
I have uninstalled and reinstalled the driver. It had been using a Microsoft supplied driver and that has been replaced by one from Intel with a date of 4/29/2018 and version 184.108.40.206, That has not made any difference.
The download connection is virtually always less than 1Mbps. A test on "computer number 2" just had a download speed of 0.67 Mbps while upload speed is 8.45 Mbps after connecting for the first time today. After disabling/reenabling the WiFi it comes back with figures of 12.48 Mbps download and 11.40 Mbps upload. The higher speeds tend to stick until/unless the laptop is taken somewhere else and then reestablishes a connection after coming home or is shutdown and restarted or the phone/hotspot is power cycled itself. At that point I'm virtually always back to glacial download speed, but the upload speed is largely unaffected.
This is not happening on two other laptops, on a HP and the other a Toshiba, that do not use Intel WiFi cards nor dual band.
As a side note, forcing the use of 5Ghz/Wireless AC makes matters worse as far as signal strength and speed goes, which is precisely the opposite of what I would have suspected.
Any theories? [I've got two nano WiFi dongles what are N300 plus one that is dual-band being delivered today to continue my testing.]