Could be lots of things. Some are related to SQL Server, some related to your environment at large, e.g. network issues, slow server in general due to non SQL Server processes, blocking on the server due to activity in msdb although you're saying there are no scheduled jobs (which begs the question, why are you trying to view job history). The issue you're experiencing is usually related to an over-abundance of job history, however if that is not the issue then there are simply too many possibilities and without having direct access to your environment to start testing some things I cannot even venture a valid guess at what it could be...good luck.