1. A method, comprising: accepting modulated symbols, which carry bits of a code word of a Low Density Parity Check (LDPC) code; computing respective soft input metrics for the bits; and decoding the code word using an iterative LDPC decoding process comprising: selecting, based on a predefined criterion, a number of internal iterations to be performed by an LDPC decoder in the process; performing the selected number of the internal iterations using the LDPC decoder, so as to estimate decoded bits and soft output metrics indicative of the input bits based on the soft input metrics; performing an external iteration that updates one or more of the soft input metrics based on one or more of the soft output metrics produced by the LDPC decoder; and repeating at least one of the internal iterations using the updated soft input metrics.