Very slow insert performance on large table (EF)
It appears that the dates you are clustered on are not being inserted sequentially.  If your date field was recent purchases for example they would be inserted at the 'end of the table'.  Because it is taking so long to do the insert it seems that SQL is having to reorder your data with each insert instead of tacking it on the end.

Determine if your clustered index is important to reads.  It is always a balance on input / output when looking at indexes.  It may be that a different column is best for clustered index.  I almost always use an integer or long primary key with no business value for the clustered index and then index dates, maybe with included columns, for read performance.

https://www.simple-talk.com/sql/learn-sql-server/effective-clustered-indexes/