Can host which is not properly set up slow down the WP? In the past year, every WP website my company did became too slow, even I noticed it when using admin p

Can host which is not properly set up slow down the WP? In the past year, every WP website my company did became too slow, even I noticed it when using admin panel for adding new features, I mean it's barely usable, constantly throwing 404 errors. Also, the clients started complaining heavily, but I'm unable to come up with the solution since on test server everything works perfectly. I also had some suspicious that's because of the bad host, because it effects every project done with WP, but the company owner claims differently since he's 'really good' at working with servers.