An official DMG of High Sierra? (downgrading from Mojave)

The only way to install an older OS like High Sierra on a mac that is running a newer OS

like Mojave is to completely erase your mac and install High Sierra.

Therefore you need to make a full backup of your mac and when you have installed

High Sierra you then restore your personal data from the back up you made,

do not restore any system or library files as this will put Mojave back on your mac.

Also any files you created or edited on Mojave may not be backwards compatible

with older versions of the apps that created them.

Why do you think your mac is slow, Mojave should not

be any slower than High Sierra.

FWIW, the High Sierra installer won't run from Mojave. You will have to create a bootable USB installer of High Sierra to boot your Mac from.

See: How to create a bootable installer for macOS - Apple Support

Why don't you download and run https://etrecheck.com

just use the free version, run it on your mac it will generate a report

which you can post back here.

Click Share Report at the top of the report this will copy it to your Clipboard.

To post the report back here click on the Additional Text button at the bottom

of a Reply window here. The report does not contain any personal data.

If you want to downgrade from Mojave to High Sierra, then you will need to perform a clean install by first erasing the drive after booting from the High Sierra USB installer. Make sure you have a good backup before performing the clean install of High Sierra.