Not specifically related to Iberia or to the T4S in Madrid, but towards the end of the year the new Entry/Exit System for the Schengen Area will enter into service, and that will cause massive congestion in all airports for non-EU passport holders. In summary, fingerprints and a photo will have to be provided on arrival during the immigration check the first time you enter the Schengen Area after the introduction of the EES - this apply to everyone that comes with a short stay visa or for visa exempt travelers. So processing time will go from 30sec to 3 min in average per person, at least for the first entry after EES.

Bottom line: do not plan tight connections for the last part of the year or you have a high risk of missing them (and IB will claim "not my problem" re EU261).