Can I downgrade my Mac to Mac OS X 10.8?

it is so slow because it does not have enough RAM memory (or its disk is dying). El Capitan needs a minimum of 6GB for appropriately-responsive operation.

That MacBook Pro can accept up to an 8GB DIMM is each of its two slots, and can be upgraded by the each DIMM to a maximum of 16GB. with 6GB or more, it should not seem slow.

Downgrading to an old, unsupported version does not provide Security Updates. A version 10.9.5 or older has no access to internet httpS: sites, which is including more and more sites these days, It will be almost unusable on 10.8.

A solid state drive will produce the most cost effective performance increases. While Mojave works with hard drives, it's optimized for SSDs.

Secure Socket Layer (SSL):

SSL 3.0 [its last version] was deprecated in June 2015 

Browsers not updated since then cannot do much Web Surfing these days.

The OP is a long-term troll, going back several years.