Speed up the site and solve the problem with "excessive number of files" on the hosting

My site is invitesandco.com slow, besides the hosting sent a warning about "excessive number of files". There are also sub-domains for customers, such as demo1.invitesco.com, which also sometimes load slowly, especially if many customers immediately enter. We need a reliable person who will thoroughly deal with everything and help with advice for the future. I would like to have experience in speeding up sites.