Without knowing your specific site's URL, setup, theme and the plugins installed there is no way for us to know.
Run your site through GTMetrix.com. The resulting report will tell you exactly what needs to be addressed.
In most cases image size (loading images whose physical sizes are larger than needed to display) and compression are the main issues.
Without knowing about your site URL, no one can say the reason of slow loading. There might be a lot of reasons to slow down your website.
Check your site speed at https://gtmetrix.com/
With Best Regard,