I participated in the development and management of a large website based on WordPress. It used to serve 6,000 concurrent visitors and reach 1 million page views per day. To make the website run fast, we had to optimize the source code of the website and the architecture of the server system. That work requires a lot of programming and system administration experience. On the system side, at least we used a separate server for the database, three servers for the WordPress source code, and a load balancer. Because your site served 1500 concurrent visitors, a sizable number, a server like you have is not enough. So you see your site is running very slowly. Since you do not have the technical experience required for optimization, I suggest a quick and temporary solution is to upgrade hardware resources. DigitalOcean has many higher Droplets to choose from.
First, let’s answer this question: What kind of resources is your server lacking? Please read this article to learn how to get performance metrics of essential hardware components such as CPU, RAM, and Disk: https://www.digitalocean.com/docs/droplets/how-to/graphs/
Then let’s analyze the last 24 hours. Any part that uses 80% or more on average then upgrades it one level. After knowing the components to upgrade, choose an appropriate Droplet. Please refer to the following article to learn how to upgrade: https://www.digitalocean.com/docs/droplets/how-to/resize/
Note: if you only need to upgrade the RAM, you can select a Droplet from the General Purpose or Memory-Optimized category; if you need to upgrade the CPU, then you can choose a Droplet from the CPU-Optimized category. However, if you do not need to upgrade the Disk, please select “CPU and RAM only.” Because if you choose to do that, then once you have optimized the source code, you can downgrade it back to a lower Droplet.
Anyway, this is just a quick and temporary solution. In a long-term, you should hire someone who has enough experience to optimize the source code and server system.
All the best,