We are using Chromium. So technically Stack is consuming the same amount of memory as chrome would do if you open the same number of apps in Stack and Chrome.
However, in Stack, after a certain time, the apps that you are not using actively, go on idle mode, reducing the load on your memory.
P.s. The information given above may be irrelevant or outdated. We will be updating the page soon. Sorry for the inconvenience and thank you for your patience!