OSX slowdown after much use

[Nintonito]

It's your device's thermal profile. The performance drop is because your device heats up extremely rapidly and throttles clock speeds to keep temperatures stable. Seeing as your laptop is over 4 years old I doubt you have any valid warranty. So I would recommend researching and acquiring high end thermal paste and look up a guide to opening the MacBook and applying the thermal paste. Those initial retina macbook's have awful thermal paste and just applying a better coat should improve things