I've found the relationship between HOLDING speed and the type of the engines, FCOM DSC 22-20-30 Flight planning / 30-10 Lateral Functions / FMS2 / Holding pattern states:
When no specific speed limit applies, the default hold speed is approximately equal to:
-Green Dot speed on the A318, A319, A320 (CFM) and A321
-Green Dot + 20 kt for altitude lower than 20 000 ft, on the A320 (IAE).
-Green Dot + 5 kt for altitude bigger than 20 000 ft, on the A320 (IAE).