Wifi suddenly extremely slow

To ameliorate the connection through the intel wifi card you can:

• Disable 802.11n
• Enable software encryption
• Enable the transmission antenna aggregation
• Disable bluetooth coexistence

Create a /etc/modprobe.d/iwlwifi.conf with the following content :

options iwlwifi 11n_disable=1
options iwlwifi swcrypto=1
options iwlwifi 11n_disable=8
options iwlwifi bt_coex_active=0

iwlwifi troubleshooting on arch-linux

TL;DR: it appears 4.17.x (and higher) kernels achieve high speeds with the iwlwifi driver with stock configurations.

I was also experiencing very slow wifi under Linux (Ubuntu 18.04LTS with it's stock kernel - 4.15.0-30-generic and 4.15.18-041518-generic) with stock configuration. My setup was capable of ~55MB/s in Windows 10, but only ~3MB/s in Linux. The card is an AC9260 in a Dell XPS 15 9570 if that's important.

I tried everything recommended elsewhere (power management, 11n_disable modprobe arguments, etc etc) and nothing worked. I reverted all those changes so my config was stock Ubuntu 18.04LTS post-install config.

I installed UKUU (an ubuntu kernel updater package, if you use a different distribution, you'll need another kernel upgrade mechanism), upgraded to 4.17.x-mainline (currently 4.17.14-041714-generic) and with no other changes I achieve 47MB/s in Linux now. My bandwidth tests are not scientific enough to say I couldn't get equal performance to Win10 now, but even in the absence of a git bisect to see what solved it, I consider this a solved problem now since a simple kernel upgrade works.

[edited 2019/01/30 to report I continue to have success with 4.19.x and 4.20.x series - it appears 4.15.x simply had a performance issue with this driver, fixed sometime on or before 4.17.x, and it has continued to be just fine since.]

My solution is to force my pc to connect to the 2.4GHz wifi channel instead of 5GHz. My wifi speed jumps to ~90 Mbps from 1 Mbps.

To check which wifi channel your pc is connecting: sudo iwlist channel

You can quickly test if this solution works by connecting to a 2.4GHz channel by connecting your laptop to a hot spot on your phone.

To force your PC to connect to 2.4GHz wifi channel: you can set different names for your different bands on your router. or find other ways on your PC side. I could follow these instructions to force my machine to connect to to the 2.4GHz channel

More information about my case: Dell inspiron, dual boot (ubuntu 18.04.2 LTS, and windows 10) wifi with win works well; wifi with ubuntu is extremely slow; connect ubuntu to the hotpot of my cellphone, wifi speed works well (this gives me a hint to find the solution).