Website down for some visitors after changing nameserver from siteground to cloudflare

[user5482]

My website is https://www.econsupplies.com

I have changed my name server from sitrground to Cloudflare yesterday. After changing the nameservers, my website is not reachable through web browser. I was told by siteground support staff that it takes few hours for dns propogation and will start working.

Its been 24 hours and now the issue is, website is loading normally for some visitors and showing below error for some including me.

in the above image it is showing lets encrypt ssl certificate, but its showing Cloudflare ssl certificate for visitors who face no issues.

Weird thing is website loads normally when i connect using mobile data and shows error in wifi connection.

[sdayman]

DNS can take as long as 48 hours to fully propagate. From my test, your site is globally available:

https://www.dotcom-tools.com/website-speed-test.aspx#A4ED7FEE913A49E988CE36CF05080E79

[user5482]

appreciate your reply. I am trying to reduce TTFB and its the main reason i chose cloudflare. But still TTFB and waiting time for requests are higher. Is there anything i can do in cloudflare to reduce TTFB and waiting time for requests or is it just bad performance by hosting provider?