Speed and utilization are not the same. My 2018 MBP was a great speed improvement over the 2015 MBP I previously used, even in cases where the 2015 MBP wasn't using all cores (which really rarely happened, since for the workloads I use, multiple Electron apps and Docker for Mac, typing in Slack would start stuttering as soon as multiple cores were in constant use). I don't think it was ever usable enough at 6-7 cores maxed for me to start new things to try to use that last core...