1. Reset your wifi password.
2. Restart
3. Connect with new password
4. Check speed

If your speed back to the optimal. Means your wifi have been hijack.

Change your wifi password into jabberish if possible. Something like "BfgHvvhj<%€?€)?(€€*;/;)&€&775(?"

Many super antenna with wifi cracking software selling in the market even in some major IT fair. Don't understand why it's even legal.