You have a few options. If you don’t want to disable any plugins, try to use the P3 Profiler plugin to identify the plugin or plugins that are slowing down your site.
If you don’t want to run experiments on your live website, you should make a copy on a sub-domain (you can use the Duplicator plugin for this), make sure the second website is not accessible for anyone (use a htpassword or a password plugin to protect it), then use the P3 Profiles or just disable the plugins one by one to see which one has problems.
You can also use the Query Monitor plugin to identify slow queries, amount of memory used, HTTP requests and other things that slow down your website.