got a Fujitsu S740 for about 40USD (used, 2.5 y.o.). It's a thinclient (similar to the T620 below) and as such sadly doesn't have Wifi-onboard (but 1 SATA M.2 and 1 E-Key inside). Otherwise beats any SBC by an order of magnitude probably. The board inside is actually sold as a "SBC" by Kontron as well...

If you are fine with older stuff and adapt a PSU, you can probably get RPI-performance (even GPIO in a real parallel-port!) for the price of a coffee.