Regression/summarizer performance collapse?

Hello, I am using many regressions in parallel over a single call to `summarize`. I've noticed that if I run ~20 regressions on a dataset with 5M rows, it seems to take 45-60 minutes to summarize. If I run a single regression on a similarly-sized dataset, however, it only takes a minute or two to summarize. What kinds of performance characteristics should I expect, and how can I avoid this kind of performance collapse?

Thank you!