The fingers of an experienced organist are capable of moving much faster than a congregation is able to sing. On the other hand, most congregations can get through a hymn much faster than a less experienced organist is able to play through their first four-part hymn.
An organist must THINK LIKE A SINGER, not like a keyboardist, when playing hymns for congregational singing. The best way to do this is to sing through the hymn in practice, and then think through the hymn before beginning to play. Note when and how long you need to breathe, and note the tempo that feels most natural.