404 error means that there is no page with this URL on the server. Most often this happens when a page has been deleted or moved to another address, but the old link remains somewhere. Also, the visitor could enter the address with a typo – accidentally or on purpose to see the design of the 404 page.
It also happens that the 404 error occurs on all pages of the site on WordPress at once. In this case, the reason is usually in the structure of permanent links. In WordPress, links have a template structure, which the owner chooses in the settings. Sometimes there is a failure in these settings, due to which a 404 Not Found error occurs when going to any page.
How 404 error affects the performance of the website
The 404 error is simply a way to report that there is no page with this address on the server, there is nothing wrong with it. It’s bad if the error occurs because it’s not the site visitor who made the mistake, but the owner. For example, changed the address of the page and did not set up redirection from the old URL to the new one.
First, the site will lose some traffic, because over time, search engine robots will remove the old page, and the new one will need time to gain positions. And it is not a fact that it will take the same place. Keywords in the text, internal and external links, as well as other ranking factors should be at least as good.
Second, instead of pages with useful content search bots will scan the page with a 404 error. This is bad because each site has its own quota of pages visited by these bots – crawling budget. If the site will have a lot of 404 pages, the search robot may begin to scan them less often, because of which you will have to wait longer for the effect of optimization. More on this topic – in the article: How the Google search robot works.
Third, a 404 error can temporarily slow down your site if you have changed the address of the page, have not yet set up a redirect, and for the old page is running an advertising campaign. Pages with errors are usually not cached, so the server will spend more resources to load it. Especially if you changed its design and now the page has more content: similar products, other articles or animations.