A Raspberry Pi 4 can manage something like 70Mbps of raw AES en/decryption flow: https://github.com/lelegard/aesbench/blob/main/RESULTS.txt

That CPU is pretty much a toy compared to (say) a brand-new M5 or EPYC chip, but it similarly eclipses almost any MCU you can buy.

Even with fast AES acceleration on the CPU/MCU — which I think some Cortex MCUs have — you’re really going to struggles to get much over 100Mbits of encrypted traffic handling, and that’s before the I/O handling interrupts take over the whole chip to shuttle packets on and off the wire.

Modern crypto is cheap for what you get, but it’s still a lot of extra math in the mix when you’re trying to pump bytes in and out of a constrained device.