It's a common issue with apps that haven't fully optimized for the unified memory architecture. You said >nothing else is really using the GPU. That's the key. During a render, your GPU should be pegged.
Open Activity Monitor's GPU History window and watch it while rendering. If it's flat, the app is using the CPU for compute. That means a software fallback, likely due to an incompatible project setting or a missing Metal backend.
Your next step is to check Pika's render settings for a "Renderer" or "Compute Device" option. If it's set to "Software" or "CPU," change it to "Metal" or "GPU." If the option isn't there, it's a bug you need to report to the vendor. A clean project file, as others suggested, is a good test to rule out legacy project corruption.