Hi, my name is Matt and can help you (22 years an IT developer, configuring and optimizing WordPress websites for 11 years).
It's hard to tell your exact issues yet many problems can be solved with careful plugin checking, optimal caching and reducing plugins/styles used.
Based on few short tests of your site you don't use best caching possible and could add some cache control for leveraging web browser caching. And there are also profiling tools to measure the performance of your website.
Do let me know. I would need a temporary WordPress access to your website to check and install&use some additional plugins. For the security you can be safe - I was a Internet Security System Engineer for 6 years in Societe Generale Group branch bank so I will advise and secure your site even further than it is now (from attacks, logins etc).
Everything will be documented with reasons why certain task was done.
Looking forward to your reply,
Matt