I have a mid 2015 27" iMac. I have reinstalled El Capitano and i want to upgrade to High Sierra.is this possible

A Mid 2015 iMac can run all the way up to macOS 12 Monterey.

see > Identify your iMac model - Apple Support

To update to the later macOS versions of High Sierra, Mojave, Catalina or Monterey,

see > How to download and install macOS - Apple Support

Yes it is possible, please refer to https://eshop.macsales.com/guides/Mac_OS_X_Compatibility to see the last version of Mac OS a 2015 27" iMac can run which is Monterey 12.X, that is what I would recommend that you upgrade to.

Then there is something wrong with your iMac, running Monterey will not make it slower.

BTW, do you have any of the following third party apps installed on your Mac?

·      Antivirus Apps

·      Cleaning Apps

·      Security Apps

·      VPN Apps

·      Maintenance Apps