Can I replace the 4GB, 1.6 GHz Logic Board with the 8 GB, 2.2 GHz?

I have a friend with an Early 2015 MacBook Air 13” whose machine is crawling now that it’s upgraded to Mojave. It looks like the 4 GB of RAM is just not really sufficient to run Mojave. Regardless, I’ve offered to install the logic board upgrade if she buys it. My question is, and I think the answer is yes, but I want to double check, can I have her get the 2.2 GHz logic board if the current one is a 1.6? The CPU speed shouldn’t matter, right?

Any answers very much appreciated in advance.