Synchronous requests stop all processing in the browser while the request is being handled. This means the user is unable to do anything further while the request is processing and may have a significant impact on response times is large amounts of data are being processed or complex routines are executed on the application server.
From a coding perspective synchronous requests are simpler to code as there is no need to handle situations where the user has moved to a different part of the application while waiting for a server response.
#Details.visible := False
#Details.visible := True
When the event fires, you might expect #Details to disappear from view, but the synchronous nature of the processing means that the property will be set to false and then set back to true before the User Interface has a chance to update.
It is also worth noting that mouse clicks that occur while the User Interface is locked will be processed when the current set of processing completes.