The numbers above coming from a test run on one of my own servers serving and receiving user generated images. It is a Intel E5 56 CPU threads machine with DDR4 RAM. A single thread may be slower then your phone CPU cores, but that's what you find very often when running servers.

I see, I can imagine a decade old CPU without any kind of video acceleration taking a while indeed. Technology has come a long way since the E5-2690v4.

Still, when encoding AVIF, there are various quality options to choose from, and the encoder has gotten better over the past year. On my i3-2120 NAS (which even your E5 should beat on single core perf), encoding an image at default speed takes about 4-6 seconds (JPEG about 2s), while producing an image of comparable quality a third the size. If I pass --speed 10, converting PNG to AVIF takes only half a second.

The standard AV1 encoder has improved a lot over a short amount of time, I invite you to run another benchmark, you may be surprised.