FileVault Encryption SSD Speed

The Intel processor has encrytption/decryption instructions built in, as does the T2 chip. Any lag is microscopic, and not detectable on human time scales. It takes far longer to transfer the data than to encrypt/decrypt it.

You should think of it as "instantaneous".

The speed of the SSD does not change, but the data do need to be decrypted 'on the fly' which imparts a lag...but not a noticeable one.

I haven't seen any measurements but I can say that there is absolutely no noticeable difference once the original encryption has completed on any Mac that I have used and that includes a 2012 base model MacBook Air through a 2020 MacBook Pro.