If the instant file initialization is not enabled then before restoring the data sql server zeroing out the data file and start process the restoration. Try enable the instant file initialization and do the restoration but you require special permission to do this.

Below link may be useful.

http://www.bradmcgehee.com/2010/07/instant-file-initialization-speeds-sql-server/

Regards,

RamaSankar

MCTS,MCITP SQL SERVER 2008.