Load speed varies on ad on website - how much is to be expected?

I have a site that has an ad whose load time varies somewhat, and sometimes it's slow to a great degree. As I start troubleshooting and working with the ad agency, it would be helpful to know to what degree a given load will vary naturally - that is, to what degree does a given load time vary moment to moment, and also day to day, for a given device based on what else that device may be doing (resources), network, CDN, or anything else. By knowing this baseline it will help convince the developers there is indeed something wrong, etc. Thanks for thoughts!