Might I suggest a referral program? If your service and quality are above your competition there, referrals should eventually take over as your main source of business. Think of ways to encourage that behavior without being annoying. Offer some cool kickbacks. Give out some apple watches. IMO, those sites are nothing but a rat race to the lowest dollar. Know your value and demand it before you take on a project.

Facebook's built-in conversion tracking does a really good job of sorting these things out. They run some complicated scripts to determine whether their ads was the origination of a conversion. However, if you really want to do your own segmenting I'd work out a system to send users to a unique conversion page based on where they entered the site. It's actually pretty easy and and would give you a definite answer as to where your best / lowest-cost conversions come from.

Jason makes a great points there, but the speed issues could also be related to the very structure of your site. Many modern Wordpress themes rely heavily on JavaScript to load things like sliders, widgets and even page content. You should use a well-supported, modern theme that is built for speed. Page caching can make a huge difference, but in a lot of cases there are 2-3 page items that should be made static content rather than dynamic JavaScript content.