Initial load is takes a lot of time in production compared to developement

I had the loading issue before and I’ve made a lot of improvements, i optimized the query issues, and now the only essential ones are made, i also put only the basic elements to load (i’m requesting the other elements that connect to DB later using ajax) to improve the initial load, the loading was indeed improved compared to before (from 15s to 6s-10s) in production and (from 15s to 2s-3s) in development, but as it figures the production still too slow, I’ve implemented all the improvement that I can on my part, i even upgraded to 4GB ram server but the same problem, i don’t have too much load on the server now 'cause i just started, Any idea what’s going on