Mojave partition APFS HFS+

I upgraded to MacOS Mojave and since then my Mac is running really slow. The "beach ball of death" appears all the time, even when I'm doing something simple like switching apps/tabs and sometimes the screen freezes for minutes. I believe this is happening because Mojave converted my HDD to APFS.

I don't want Mojave or Bootcamp deleted, I just want to install High Sierra in a separate HFS+ partition. Is this possible?