Mullvad VPN and battery drain

I got Mullvad VPN to try it for a month after years using ExpressVPN, but it drains my battery (iPhone SE, 2nd generation, 2020) into oblivion in a matter of 3-4 hours.

Browsed the Cursed Site™️ and using WireGuard instead of the Mullvad VPN app seems to be the most common solution – but not for me. WireGuard either it doesn’t connect to any location or disconnects after 2-3 minutes.

Anyone got any advice?

EDIT: added more details about the phone model.