It's either your theme or a custom extension.
Try the following:
- switch to a luma theme and check speed. If it's better than it's your theme files.
- turn off all 3rd-party extensions and check speed. If it's better try turning on extensions one be one and see which one breaks performance.
You could try code profiles, tideways, new relic or similar.
I wrote a special article on how to improve Magento 2 speed - check it out.
