What is the problem if radiators (oil heating) take a long time to get to full temperature?

The house is an old stone cottage with a modern extension. It has 2 fireplaces (1 blocked off) and 1 with an old fire and back boiler that can also heat the hot water tank (and radiators when you turn on a switch).

There is oil central heating. Any oil heating I've experienced in the past goes on, and within a shortish time frame the radiators are nearly too hot to touch. In our cottage, if the oil is on for 1 hour, the radiators are hot, but you could hold your hand on them no problem. It take about 2 hours before they reach the temp you'd expect.

The boiler has been serviced, rads have been bled, so I'm thinking that the rads are filled with sludge and need taken off and cleaned out?