The subscene used for the first test was 95% on subswath IW3. So I ran the same original graph, adding TOPSAR-Split of IW3, and measured these times:
v8.0.0: 16.2 real, 184.2 user, 25 sys. Peak CPU utilization ~1700%
v7.0.2: 7.1 real, 89.8 user, 7.1 sys. Peak CPU utilization ~1700%
I started testing the graph split in half, but for some reason 7.0.2 never completed the Terrain-Flattening half. I will post additional results soon.