Sql Server Restoring Process Time Takes Too Long Time
As long as you state that you did restore before and it took much less time I would quess that some of the following could be the reason:


some other process on the reporting server taking a lot of resources (in- and outside of SQL Server), that put high load on disks;
slow network connection between backups and reporting server if backups are located on a network share;
some process loading the disks with backups preventing fast reading of them by SQL Server.