Why does my 2009 Hyundai i30 struggle to go past 1500rpm when in gear?

There are many reasons this could happen, and they could be mechanical or electrical. An engine that revs freely with no load on it, but goes weak at the knees when a load is placed on it, could be suffering from poor fuel supply, a faulty ignition system, internal wear, a blocked exhaust or even a worn driveline that is creating excess friction. You could even have a seized brake or something equally fundamental going on.

But it’s also true that the car’s computer can send the engine into what’s called limp-home mode if it detects a serious problem. Limp-home mode limits the amount of power the engine can make or how fast it can rev, as a means of protecting it against further, more serious damage. Perhaps that’s what’s going on here. Certainly the symptoms match that diagnosis. The best advice is to have the car electronically scanned to see what fault codes are thrown up by the computer. Only then do you have a decent starting point on where to look for the root problem.