We always strive to keep our tools swift and resource-efficient, and the current implementation of Firewall should have minimal impact on the battery.
If you decide to review the battery usage in iOS Settings > Battery, it's okay to see a high background time for 1Blocker because Firewall works via a local VPN profile. The thing is that if a VPN profile is enabled, it's considered active by iOS. It shouldn't affect your battery, though.
The battery usage rate may be a bit higher than usual if the app actively blocks tracking-related requests because each request triggers Firewall.
In this case, it's worth setting Firewall's filtering mode to DNS Proxy in the (…) button at the upper-right corner of the app's Firewall tab. 1Blocker may take about one minute to reconfigure the filtering mode.
Note: it's recommendable to opt for longer time intervals while checking battery usage, such as the Last 10 Days tab in iOS Settings > Battery.