Cobravila, in my experience the Idle performance factor on the status page (as per WhyBeFlier's post) is an important part as to wether the descent profile is accurate. The company I fly for has IAE powered A320s and A321s. In the past on a managed descent the A320s were always slow and the A321s were good. The idle factor was changed across both types. the result was the A320s good and the A321s always ended up way too fast. The company fixed the A321s idle factor and they began reasonably good.
Another issue I find is that on the A320 is that when you modify the descent speed on the performance page (before descent starts), if you you use 280kts, it flies the managed descent at a reasonable 280kts. If you try it with 320kts, the managed profile will usually end up requiring a speed of 340kts.