The whole point is that it's unusable without Google services due to battery drain. Meta got it right with WhatsApp and a battery-efficient WebSockets implementation for notifications without Google services, but Signal's implementation is poor.
I have run Signal on LineageOS or GrapheneOS for nearly a decade, no Google services, and I have always had satisfactory battery life, i.e. ~2 days of normal use.
I bought a PinePhone, so I definitely know what truly unoptimized battery life looks like, lol.
This was never an issue with my Pixel. Using my pixel with graphene without google play services resulted in better battery life than stock.
This is an exaggerated take. I've been running Signal on LineageOS for years across at least 5 different phones in exactly this manner with no Google services installed. I'm currently using a Motorola G84 and charge every 2 days with moderate usage; Signal is my primary communication method with family.