It is really a strange issue which has not been reported by other clients. A possible cause for the issue can be if you are using Cassini web server which is the default built-in web server of Visual Studio since running the project in Cassini is always slower than running the project in IIS. This is because the built-in web server have a limitation on the amount of hardware resource it uses.
What I can recommend is to replace the Cassini web server with IIS express
which is also started from Visual Studio. Please refer to the screenshot
Can you please try this out and let me know if the issue still persists.
Do you want to have your say in the Sitefinity development roadmap? Do you want to know when a feature you requested is added or when a bug fixed? Explore the Telerik Sitefinity CMS Ideas&Feedback Portal
and vote to affect the priority of the items