Bing Maps works fine at first, then gets slow

I've been trying to get Bing Maps layers to work in ArcMap. I have a Bing Maps key, I load the layers using the "Add Data From ArcGIS Online" command, and it works great at first. Response time when panning and zooming the Bing Maps layer is good.

But after a while the behavior suddenly changes. The response time when panning and zooming the Bing Maps layer gets much worse. It still works but it's so slow as to make using it tedious.

I ran Fiddler to analyze the HTTP requests ArcMap was making. When I first load the Bing Maps layer it makes requests to virtualearth.net like I expect. But when the slowdown occurs I see a lot of requests to serverapi.arcgisonline.com which weren't occurring before. The request path is always /veadaptor/production//services/identity/clientip and there are two such requests for every tile request, a HEAD followed by a GET. I think these requests to serverapi.arcgisonline.com are what slows down performance so much.

At this point I save my project, exit ArcMap, then run ArcMap and open the project. Performance of the Bing Maps layer is great again until some time passes and those requests to serverapi.arcgisonline.com begin again.

Does anyone know why this happens? Why does ArcMap relentlessly query serverapi.arcgisonline.com for every tile request to Bing Maps?