Wlan on the pc suddenly very slow?

However, I recently had the problem that the Wi-Fi on my notebook is no longer working properly or only slowly with a download speed of not even a megabyte. What can I do? Have already changed the network card after a few minutes then it did not work again

lg. Tim

Some possibilities have already been mentioned. I'll add:

Bluetooth can interfere with 2.4 GHz WLAN, sometimes switch off Bluetooth

(Antenna cable on the notebook's WLAN adapter (you apparently fiddled around to change the Wi-Fi card):

Antenna cable plugged in correctly?
The antenna cable has a broken cable (often runs over / on the hinges of the lid)
DSL router is crazy, start again
WLAN repeater or dLAN / Powerline adapter bad, test it with another WLAN device (smartphone). Or test it directly on the router using a LAN cable.
Weird software installed? Antivirus programs, … Then deactivate or uninstall.