WordPress sites may lag or perform slowly due to several reasons, mostly related to improper optimization. Here are some of the main causes:

1. Heavy Themes and Plugins:
   Many WordPress themes and plugins come with a lot of features, which can slow down the site if not well-optimized. Poorly coded plugins, in particular, can significantly affect performance.

2. Hosting Issues:
   Low-quality hosting services can cause WordPress sites to lag. When site traffic increases, insufficient server resources can lead to slowdowns.

3. Caching Problems:
   If caching is not properly configured, the website has to fetch data from the server every time, leading to slow page load times.

4. Unoptimized Database:
   Over time, WordPress databases can become cluttered with redundant or unnecessary data. An unoptimized database can cause the site to slow down.

5. Uncompressed Images:
   Large, high-resolution images that are not compressed can slow down page loading times. Failing to optimize images can significantly affect overall site performance.

6. No Content Delivery Network (CDN):
   Without a CDN, content may load more slowly for users who are geographically far from the server. CDNs distribute content across servers globally, speeding up the loading process.

7. Lack of Proper Caching and Minification Tools:
   Minifying JavaScript and CSS files and removing unnecessary code can help improve performance. If these practices are not followed, the site can be slow.

To solve these issues, it's essential to apply optimization techniques such as choosing lightweight themes and plugins, compressing images, cleaning up the database, and using a reliable hosting service. Implementing a CDN and effective caching strategies will also boost the site's speed.