1) what are the IO stalls on your disk system, especially writes? see sys.dm_io_virtual_file_stats in BOL
2) as others have mentioned, indexes/triggers. also unindexed FKs maybe?
3) growth rates on the file?
4) network latency if loading from different machine?
5) IO config in general - sector alignment, ntfs cluster size, number of spindles, controller/SAN cache ratios and queue depths, etc?
6) what stuff have you already done or checked on? If you aren't a pretty skilled DBA I will recommend you get a professional on board for a bit of tuning/mentoring. TB sized databases take special handling and knowledge to make them efficient
Best,
Kevin G. Boles
SQL Server Consultant
SQL MVP 2007-2012
TheSQLGuru on googles mail service