No way to know for sure why you're suddenly seeing slow performance. Can you gather more metrics? Take a look at sys.dm_exec_requests. Can you see the backup process? Is it blocked? What is the latest wait state on it? Does it show a percentage complete? This information will help you troubleshoot what's going on.
"The credit belongs to the man who is actually in the arena, whose face is marred by dust and sweat and blood"
- Theodore Roosevelt
Author of:
SQL Server Execution Plans
SQL Server Query Performance Tuning