Yeah its really quite slow. Anecdotal evidence suggests its slowerin 6. 4x using construction brigades than it used to be in 5. 5. 4 with the engineers.
3 units is about the minimum I will commit to these ruins now, anything less and it can takes many years to salvage the site. With 5 or 6 construction brigades i start to see simultaneous recoveries.
Even then. . . its slow. I found a pair of ruins 1 and 2 jumps from Sol in my latest game, one TL3, the other TL4. By the time I had designed and constructed a large-scale troop transporter and actually get teams into place to exploit these ruins I was already entering TL5 and the ruins were able to provide nothing more useful than mines/factories.