php - performance issue in website based on LAMP architecture -

I have an internal website that uses LAMP architecture. My main page takes approximately 10 seconds to load data. There is not a lot of data around 4-5 records. I do not have any complicated MESQL queries, but there is a lot of them, that is, around 10-15 questions. Actually I'm removing meta-data to display on the page. These are very simple questions. I have a lot of PHP and JavaScript logic, which is of medium complexity. I can not remove any of these I have around 1800 line codes in that page and I am using data to display data. The stats contain 25 columns and many HTML selection elements. So how do I know why this page lacks performance? I tried to be as clear as possible, but please tell me if you have any questions.

Appreciate your time and help.