Long waiting time in 301 redirect on WordPress pages

I have a WordPress website and the GTMetrix analysis is as below:

https://gtmetrix.com/reports/www.insil.com.au/NrpADawD/

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The waiting time of the request is almost 600ms and in other attempts it reached 1.5 seconds just to add trailing slash.

I tried add Redirect 301 /build-a-website /build-a-website/ but it caused a loop redirect. Any idea how to reduce the waiting time or identify the cause of it?

If it's WordPress that's adding the trailing slash, WordPress core and plugins may be loaded, and that's what slows down the redirect time. Some hosts have an interface for managing redirects, that give you the highest performance possible.

In general, using htaccess for redirects can also cause a performance impact, so be aware of that.

For your explicit redirect, try changing it to this:

Redirect 301 ^/build-a-website$ /build-a-website/

The caret ^ specifies the beginning of the string, and the dollar $ specifies the end of the string. This means that /build-a-website/ will no longer match to /build-a-website because it is explicitly indicated that the e of website is the last character of the URL.