If I try to delete posts manually then the process get’s killed after it deletes about 30-40 posts and it takes about 2 minutes to delete those 40 posts.
After that I get a screen from CDN saying that the server is not able to respond. It takes another 2-3 minutes before I can access the admin again.
If I set it to just delete 10 posts, then it takes about 30-40 seconds to delete just 10 posts. If I increase the number then the process takes longer and get’s killed.
That’s why I was saying that your server is under load. It need not be that your specific WordPress installation is under load, but the server where your website is hosted might be under load because of other websites that are hosted together with it.
Does your webhost provide any type of dashboard where you can check the load status of your WordPress installation and server?