How effective is Readyboost with an SD card for enhancing the performance of an old laptop with 8GB of RAM?

I have a Dell XPS13 laptop that is 6 years old and has a new battery and SSD, but the RAM is not upgradable. I want to extend its lifespan as much as possible, so I am wondering if I can use an SD card with Readyboost to boost its memory. I have heard that Readyboost is only useful for systems with very low RAM, such as 1GB. Is this true? How much difference would an SD card with Re