What can cause the high ‘initial server response time’?

I’m working on page speed optimization of a wordpress blog and I’m using pagespeed insights tool for it.
It’s showing high ‘initial server response time’ of 600 ms to 900 ms.

My GTM had 4 to 5 tags running. For the experiement, I paused all of them and kept only empty gtm running. It was showing initial server time of 600 ms. Then I enabled UA script only which trigges on ‘dom-ready’. My GTM itself is delayed by 1700ms. Now, it shows initial server time of 900ms.

The other factors are as below:

1. I’m using vultr’s high frequency cloud compute shared plan of $6.
2. My server is located in US
3. The daily traffic is hardly 20 users/day
4. I’m using opnelitespeed server and litespeed cache plugin for optimization and caching purpose
5. I’m using cloudflare as CDN
6. Base theme is astra
7. Most of the custom theme is hard coded and optimized for speed.
8. GTM has UA tag, GA4 tag, MS clarity, pinterest tag and GA4-test tag
9. For css, one css file of main theme, then 1 or 2 page specific css files for child theme. They are not showing any delay.
10. There is only one js file for my site. It is minimal and coded in vanilla js. I’m not using any library like jquery.
11. There are average 4 to 6 max images on each page but none of them is above the fold and they’re lazy loaded. The average size of each image is around 80kb **but wordpress seems to be scaling them up to 120 kb. I don’t know why.**
12. The number of requests range from 27 to 31 max.
13. The high sized files are third party scripts like UA which average around 120 kb.
14. HTML page size is below 1 mb.

If scripts or images were the cause, they would have contributed to high Total blocking time, or time to interactive.

TLDR; I’m not able to understand what’s causing initial server response time as my html is not heavy, base theme is astra, scripts are delayed, images are optimized and lazily loaded, server isn’t very basic, caching and cdn are implemented.

What can be the reason?

 

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