Macs have encryption in hardware in the dma channel for their built-in drives (Intel Macs with T2 and all ARM Macs), so the overhead is negligible on the internal ssd. Macs actually don’t even have unencrypted internal drives anymore. The filevault toggle only affects whether the volume encryption key stored in the secure enclave is itself encrypted or not.

Older Macs and external drives are a different story of course.