SQL Update Efficiency and Blocking Concern
Table locking is a good thing, it's done so your data doesn't get corrupted.

I wouldn't recommend using NOLOCK when writing data, you should have a look at the answers to Is the NOLOCK (Sql Server hint) bad practice?

An article linked from the question (SQL Server NOLOCK Hint &amp; other poor ideas) has as it's first recommendation:


  Do not use any HINTs until your testing proves that you have an issue
  that can’t be solved any other way than by using a HINT.


As ypercube commented, I would work out why your SELECT query is slow before applying lock hints to you query.