I am just going to do a copy and paste:
When I go to some forums, the forum will sometimes load fast other times very slow, the same with posting in the forums, can advertising slow down the site?
Yes, advertising can indeed slow down website loading times, including forums where you might experience sometimes fast and sometimes slow performance, both in browsing and posting. Here's a explanation of how and why this happens, along with some other potential factors.
How Advertising Can Slow Down Websites

Asynchronous vs. Synchronous Loading: Well-implemented ads using asynchronous loading (e.g., with async or defer attributes in their script tags) allow the main webpage content to load without waiting for the ads. This minimizes impact on the initial page render . However, if ads are loaded synchronously, the browser must wait for each ad to fully load before continuing, which can significantly delay page rendering .

Volume and Size of Ads: A large number of ads, or ads with heavy media files (like high-resolution images or videos), increase the amount of data the browser needs to download. This consumes more bandwidth and processing power, leading to slower page loads .

Third-Party Scripts and Networks: Ads often come from third-party networks. Each external ad call requires a DNS lookup, establishing a connection, and transferring data. The performance of these external servers, their geographic distance from you, and network congestion can all introduce delays . If one third-party script is slow, it can block other elements from loading.

Ad Refresh Techniques: Some websites implement ad refreshes every 30-240 seconds without reloading the entire page. While this can keep ads relevant, the process of fetching and rendering new ad content can consume resources intermittently, potentially causing stalls or lag during your browsing session, especially if you're typing a post .

Layout Shifts and Rendering Blocking: Ads that don't have reserved space or load dynamically can cause Cumulative Layout Shift (CLS), where page elements jump around after initial render. This creates a poor user experience and can make the page feel slower as it becomes unsteady . Some ad scripts might also be render-blocking.
Other Factors Affecting Forum Speed
While advertising is a common culprit, the fluctuating speed you describe (sometimes fast, sometimes slow) can also be due to other factors:
Server-Side Issues: The forum's server might be under heavy load (e.g., high traffic, insufficient CPU/RAM), have slow database queries, or lack proper caching mechanisms . This is a very common cause for slow posting, as submitting data (a post) is more resource-intensive for the server than simply retrieving data (reading a thread) .
Network Congestion: Your own internet connection, your ISP's routing, or the network path between you and the forum's server could be experiencing congestion or intermittent problems .
Client-Side Factors: Your own computer's available memory, CPU usage, or browser having many open tabs/extensions can also affect how quickly pages render .
What You Can Do
Use an Ad Blocker: This is the most direct way to see if ads are the primary cause. If a forum loads consistently faster with an ad blocker enabled, you know ads were a significant factor.
Browser Hygiene: Keep your browser updated, close unused tabs, and disable unnecessary extensions .
Check Network Connection: Try accessing the forum from a different network (e.g., switch from Wi-Fi to mobile data) to see if the issue is related to your local network.
Summary
Advertising can definitely contribute to slow and variable loading times on forums and other websites. The impact depends heavily on how the ads are implemented and the volume of ads. However, server-side issues are also a very likely cause, especially for slow post submissions. The intermittent nature of the problem suggests a combination of factors, including potential network congestion and the variable load from third-party ad networks.
To quickly test if ads are a major factor for a specific site, try using a reputable ad blocker and see if performance improves noticeably.