Optimize backend on web server for Live ios app and scale

Currently have a live app in the apple ios app store. The back-end is running off a VPS server from Godaddy. The social media app is really slow. The like button, comment and notifications are slow and need to be instant. The trending category also needs to be tested and fixed. (All this can be done on the backend) There is a server 500 error and a network error sometimes.. mainly because there is not enough RAM running the server. I need someone to optimize the server. We require someone to optimize the GoDaddy VPS Server to ensure there are no delays and no lags for up to 2500 people using the app simultaneously. You will be expected to provide proof of changing the speed. I need this server to be very cost efficient. I need a cost estimate of 2500 people using the app. I will need help understanding how to scale the app once more people start using it. If you to move to a different server for a specific reason, i will be open to it. I need your help to scale this app in the most cost efficient way. Looking forward to work with you, thanks!