Do you have Folders or Files on your Desktop? If "yes" then the most likely cause is that Windows has to gather all the info relevant to these Folders & Files, which could take some time depending on the number and size of the files. The second time you click on the Tab the information appears immediately because Windows has already collected this info and stored it in a .tmp file.
Solution. Your Desktop Icons should only be shortcuts to the Files. Shortcuts are only 1KB and can be collected very quickly.