Yes, WordPress website is slower than HTML, because Website build on WordPress is dynamic compared to HTML which makes static websites. When compared, a static website will load faster that is HTML based website will load faster than a dynamic website made from WordPress. Added to that WordPress is written in PHP, and all its files take more time to get executed than HTML static files.
Wish to learn more then check out the Full-stack development course from Intellipaat which provides online training course apart from exercises and guided projects.
Also, do check out our YouTube video on HTML vs WordPress to help you get started.