I have tried my best to optimize my web pages load time between 20 ms to 30 ms. Every page opens in this time frame but when I see the results on webmaster it says my web pages are taking around 250 ms to 300 ms to download a single page. Since we are using PHP and I have also installed brotli (instead of gzip) on my Nginx server.
what else can be wrong ? is it like network congestion or still something wrong with my server ?
While I admire your aim, a load time of 20 ms to 30 ms is just not achievable. 200 - 300 ms is amazing.
When you are measuring, you are probably measuring on a warmed up connection and possibly right besides the data centre. Webmaster (by which I presume you mean Google Search Console?) is not so is likely doing:
And those “typical” times very much depend on the type and distance of the connection. Again you are probably measuring on a fast PC with a fast internet connection that is already warmed up with steps 1-3 whereas an independent scan like webmasters will not be an may be simulating a slower connection of an average user.
Run your page through https://www.webpagetest.org and ideally share the link to see if there are other opportunities to improve performance.