Hi, I've optimized probably more than a 100 WordPress installations in projects just like this and I am confident that I can help you because I know everything that could cause issues with the speed, server side and WP side.
I will need from you not only WP Admin Access, but more importantly SSH and FTP access to debug the server side too and understand exactly what is missing and what can be added.
Check my previously completed projects here related exactly to this type of thing. Last one completed a couple of days ago by doubling the website speed:
https://www.freelancer.com/projects/Wordpress/Improve-WordPress-Site-Speed-11813866/
Please understand that the bid for applying the changes is just an educated guess and not the final value, but I would ask you $80 - $100 if I have to work only on the WP side by fixing everything WP related.
If I have to do changes on the server side, it will cost even more, probably $150 - $200 in total including the previous changes, because on the server I have to consider multiple factors like PHP version, Apache version, etc and I need to be sure that there are no other websites that might be affected from the server changes, so there is more time needed.
Looking forward to your reply.
Contact me in chat so we can discuss.
Regards,
Ermal