I don't read Parterre, but as I said, I'm almost certain they are WAY off in estimating only 20 minutes of cuts from PORGY. Here are my estimates of the timings of the major cuts:
1. Jazzbo Brown Blues -- about five minutes
2. reprise of "A Woman is a Sometime Thing" plus other minor cuts in the first scene - at least seven minutes
3. the Mr. Archdale scene, "The Buzzard Song," and several other small cuts in that scene -- at least 10 minutes
4. several cuts in the final scene, including most of the orchestral introduction, the children's chorus and the sequence where Porgy gives presents to his friends -- about 10 minutes
So even if I'm overestimating the timings a little (and I don't think I am), that's about 30 minutes right there, PLUS there are several other cuts in other scenes that I haven't even mentioned.