Using a tool such as YSlow, Firebug or one of the many others that provide an analysis of the total page load time can often help you to see and understand the difference in performance between the time required for the web page to load vs. the time required for all the banners to load.
Note: This issue can be particularly exacerbated by the SPC and the Asyncrhonous Tags
Step 3: Review Server Performance
Poor performance on the Revive Adserver server (in general terms - not having enough memory, CPU or disk I/O performance to meet the load demands of the server under load) can result in poor banner creative load time and/or lost data, as the server fails to respond to e.g. impression beacon requests, and users' web browsers time out. This can also lead to "missing" impressions when compared with the request levels.
Step 4: Consider Tag Type
The issue of slow loading banner creatives and/or poor server performance can be particularly exacerbated by the use of the SPC Tag and the Asynchronous JS Tag, because these tags are specifically designed to allow web pages to fully load first, and then load the banners afterwards.
While allowing the page to load first, and then loading banners afterwards is a good thing for the user's experience, if either of the above two issues (slow loading banners and/or poor server performance) exist, then these tags will make the problem much more obvious, as users "navigate away" from pages before banners finish loading.
If you absolutely cannot address the creative load time and/or server performance, you may want to consider using another tag type. This will give the website user a worse experience (that is, an overall slower page load time) in exchange for a higher number of banner impressions.