This article includes a video encoding comparison between three different Macs with T2 chips, a Mac mini, an iMac Pro, and a 13" MacBook Pro: The 2018 Mac Mini. Naturally, the iMac Pro was the fastest when encoding using the CPU but the HEVC hardware encoder in all the T2s was dramatically faster and identical on all models. Here’s another article that compared a T2 Mac mini with an iMac without a T2 that includes more detail for how they configured the software for testing: Apple’s T2 chip makes a giant difference in video encoding for most users.
As the this TidBITS article notes, the T2 is based on the A10 ARM processor found in the iPhone 7. It makes sense it contains such video hardware acceleration since a phone would need such dedicated hardware to record and playback such videos, to save battery life and keep the phone from melting in your hands 