Internet speed slows down over time?

When I turn the wlan on and off at the laptop, I have about 15 Mbps. After 5 minutes I'm under 2 Mbit / s. The router is not, which has already been reset.

Download in the background? Others in the house which burden the line?

No both excluded

Could also be a broken network adapter. Do you have a WLAN stick at hand? Or n LAN cable?

Lan cable but not Wi-Fi stick

OK. Stick in, test, report

Have my at least 10 years old Wlan stick unpacked, which even creates the 16Mbit we have, now it's back to how it should

Then it is a defect of the adapter. If it works again then it's perfect