When referring to slowing down speed: are you referrin to that column time/iter?
Please do not rely on that if yes. The ultimative test will be to run the case via journal in batch and introduce some time stamps to compare between certain parts in the journal. You might there also rely on the benchmark command or just create scheme variables to store time before and after and it prints you the acculated time. Slow down might have several reasons: software, OS and hardware!