Extremely slow wifi on Ubuntu 20.04 after standard solutions, 18.04 is fine

I have a clean Ubuntu 20.04 install on a Thinkpad X1 Carbon. I am running the kernel 5.13.0-28-generic. The wifi is extremely slow, making e.g. Zoom impossible, dropped calls, total loss of connectivity. With a speed test I get values of 7 Mbps Download, 5 Mbps Upload (and often much worse) while a different Thinkpad (X390) running Ubuntu 18.04 and sitting next to the X1 gets 52 Mbps Download, 44 Mbps Upload from the same home Wifi and the same speedtest. I even booted the computer with a USB stick under 18.04 (kernel 5.4.0-84-generic) and got 53 Mbps download, 37 Mbps Upload on the same computer!!

I read through similar questions and tried all solutions I could find.

1. I have disabled the 802.11n protocol adding options iwlwifi 11n_disable=1 to /etc/modprobe.d/iwlwifi.conf -- No effect.

2. I have disabled power management by editing /etc/NetworkManager/conf.d/default-wifi-powersave-on.conf and changing the value from 3 to 2. I confirm by iwconfig that power management is off. No effect.

3. I have edited /etc/nsswitch.conf and changed hosts to hosts: files dns -- No effect.

4. In desperation I have edited etc/gai.conf and uncommented the line Precedence ::ffff:0:0/96 100 -- No Effect.

5. Also, backport_iwlwifi_dkmsis not installed.

Help would be greatly appreciated. Any ideas?

EDIT: Output of lspci -nnk | grep 0280 -A3

00:14.3 Network controller [0280]: Intel Corporation Device [8086:a0f0] (rev 20)
    Subsystem: Intel Corporation Device [8086:0070]
    Kernel driver in use: iwlwifi
    Kernel modules: iwlwifi

Check your WiFi MTU, using

ip link

or

ip l | grep $(ip r | awk '/default/ {print $5}' ) | awk '{print $2, $4, $5}'

also notice your WiFi interface's name.

The MTU (Maximum Transmission Unit) is the size of the largest packet that can be sent in a single network transmission. If a packet exceeds the MTU of a link, the data must be split into multiple packets (fragmented). These multiple packets must be sent over the link, received, acknowledged, and reassembled at the far end. If your link is misconfigured, and you have to fragment every packet you send, your actual data transfer rate drops.

Ethernet (wired) networks use an MTU of 1500 bytes.

Due to additional per packet overhead for WiFi (8 bytes PPPoE header), WiFi uses an MTU of 1492.

Your MTU should be set by your DHCP server, check your router's config.

You can set your own MTU (setting does not persist over restarts) with

sudo ip link set dev name mtu 1492

where "name" is the interface name from above.

Here's an example:

walt@squid:~(0)$ ip link
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN mode DEFAULT group default qlen 1000
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
2: enp63s0: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc mq state DOWN mode DEFAULT group default qlen 1000
    link/ether 00:24:21:7f:e5:1c brd ff:ff:ff:ff:ff:ff
3: wlxf46d04b1790f: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc mq state DOWN mode DORMANT group default qlen 1000
    link/ether f4:6d:04:b1:79:0f brd ff:ff:ff:ff:ff:ff
walt@squid:~(0)$   sudo ip link set dev wlxf46d04b1790f mtu 1492
[sudo] password for walt: 
walt@squid:~(0)$ ip link
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN mode DEFAULT group default qlen 1000
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
2: enp63s0: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc mq state DOWN mode DEFAULT group default qlen 1000
    link/ether 00:24:21:7f:e5:1c brd ff:ff:ff:ff:ff:ff
3: wlxf46d04b1790f: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1492 qdisc mq state UP mode DORMANT group default qlen 1000
    link/ether f4:6d:04:b1:79:0f brd ff:ff:ff:ff:ff:ff

My WiFi "interface name" is "wlxf46d04b1790f".