this kills the battery though

Does it? I've been using it on and off, on Android. I have set it to disconnect when screen is off. The impact on battery does not seem to be drastic, though it is difficult to do real side by side comparison. Another option is to only use it on demand, while browsing.

Is there any other option?

I wonder why. Storage disk encryption does not, neither does SSL. Why are VPNs using much more power on mobile?

>Why are VPNs using much more power on mobile?

It can leave connection open which prevents many cellular radio technologies from falling into lower power state.

Really depends on how optimized the cell modem is and how often keepalive packets are being sent by the VPN. Honestly, I haven't seen too much of a battery hit on ipsec VPN on the iphone SE. However, my older android phones used to get shredded on OpenVPN.

EDIT: If anyone's asking, I use the algo.sh script to deploy ipsec vpn on digitalocean (has option to enable adblocking too) https://github.com/trailofbits/algo.

Wireguard should alleviate exactly that.

It's designed to not use persistent connections, keepalives may have to be enabled in case of nat traversals though (to enable incoming packets).

https://www.wireguard.com/quickstart/