Liew Cheon-Fong

you can find plugin to limit user login on single device (it uses login info + cookies to detect)

you can use anti spam plugin

try if($post->post_type == 'page') { /** your code **/ }

try a email marketing plugin

there are two empty <p> HTML paragraph tags in between.
Check your mailchimp form and remove them

screenshot: https://i.imgur.com/kr32tl7.png

most probably your hosting is slow to response (thus the timeout error) or hosting account does not have enough resources.

It only affects you (admin) in the admin page because admin pages are not cache and loading pages trigger more PHP process and MySQL queries.

Front end is not affected much if it is cached.

A user’s process timed out does not mean other users have the same issue with difference process.

it is common mistake if your website is sharing hosting server with others (same IP address). Try to contact your hosting provider about the issue