Why does my website load slowly and how can I fix this issue?

My website has been loading very slowly lately, especially on mobile devices, and it’s affecting user experience and SEO. I’ve tried basic optimizations, but the issue still exists. I recently came across a web development company as a possible solution for performance and optimization issues. Has anyone faced a similar problem before? What methods or tools worked best for you to fix slow website loading?

Note: link to company removed by moderator as spam