A little more detail might help you get an answer
What specific version are you running of MERN? NodeJS? Anything else relevant?
On what kind of machine? Local on your laptop / desktop? Shared host? Cloud host? AWS? Digital Ocean?
If it's a cloud host, size of server instance? If it's a local machine, specs of the machine?
Have you changed or made any specific configuration settings to your app? Some sample code (if possible) of any initialization scripts might help someone, help you.