The site is built based on nopCommerce v3.6, it used to run OK, but recently it's been slow and slow, particularly the homepage, we haven't done anything complex.
Hi I have tested your site in the webpagetest tool, here I can see your main problem is time to first byte. It's too high. (There might be many reason behind this) Can't predict blindly. One more thing please Enable JavaScript and CSS bundling from admin site to increase front end site speed.
Hope this helps.
P.S here you will find some tips for performance improvement in performance tab. Please enable those settings and try.
Yes, nopCommerce team has done lots of work for performance optimizations since last few versions. So obviously your site performance will increase with newer versions, however it depends on some configurations, server response, number of products, your own customized work, hosting companies, your front end optimizations etc.