Why TTFB and Content Download time is higher?

Question

I have an ASP.NET MVC Application. But one of the page is taking so much time to load. I have used jQuery datatable to render the data in that page. I am not using backend processing though I have plan to move into backend processing (like, will load only the current page data). Currently, the page is rendering around 18,000 rows and each row has 15 columns.

TTFB: I have found that TTFB is around 30 seconds which is very high. I have also checked whether the backend is taking too much time or not. After debugging, I have found that backend processing is taking utmost 4 seconds. So how can I minimize this TTFB time?

Content Download: With the 18,000 rows, I have found that the page size is around 5 MB and there are other scripts and CSS whose sum of size is 1 MB. But it's taking around 1 min to download the content though my internet connection is fast. So can you please suggest me how to minimize this time?

Please note that I am getting this time in localhost and also in server and the server is Azure App Service. Please let me know if you have any suggestion.

Thank you.