I noticed that when just idling, it was using 9 Watts, when I plugged the phone in, it jumps to 14.3. Now maybe I am wrong here but 5.3 watts/5.05 Volts should be just around 1 amp?? It easily takes twice as long to charge with the psu hookup than the 1 amp wall wart I was trying however.
The ATX PSU is most likely running well below 50% efficiency at low power outputs. You say it is old – if it is anything other than reputable maker (FSP, … there are several) or more than 2 years old it may be around 20% efficient at that low power.
BTW – I too am planning to power my Pi from my PC. But figure on getting some pins onto the GPIO place and jumpering 5V straight in. Figure one of the old FDD 3-pin headers looks perfect with the middle pin not connected (on the Pi side).
PS - as per shiftyphil having a load only on 1 rail of an ATX PSU is likely to be a poor config: many more recent PSU specify a min load on 12V (and 5V) to get them to run the way they were intended.
Daily driver: Pi3B, 64GB Samsung Evo+ @100MHz, DVB-T, onboard WiFi for internet, BT/USB dongle for KB/mouse, 250GB HDD via USB for media, Raspbian Jessie Lite with Openbox desktop.
Museum: Pi B