The amount he wasted during the transformation was particularly high. It was as high as 1.15 million out of the 1.25 million years. The wastage could be less, but True Loki let Black Loki be so wasteful intentionally.
He thought it might be odd if Black Loki was extremely efficient and proficient at making copies right from the start. So he left room for improvement, so that Black Loki could get better and better organically.
Clearly, it wasn't only Black Chain doing something to deceive his warden and captors about how difficult what he was doing was. Even though Black Loki was taking all of the credit, it was still a team effort.
Another thing they had to sacrifice to make their pretense believable was the progress of their conversion efficiency. Because Black Chain was so busy that he had to close his eyes and shut off his senses to concentrate, he couldn't grind his conversion efficiency.
