One thing you can try is reducing the encoder effort: libjxl produces something like 5% larger files, but at 3-6x faster encoding speed, if you turn it down from 7 (default) to 4. (The typical compression rate benefit vs. JPEG is 30-60%.)

Shameless plug: if you have GPUs in your server, you can put them to use to make it faster! https://github.com/yunho-c/gjxl (Granted, the speed improvement at lower efforts aren't as pronounced)