PageIOLatch = contention on the IO subsystem. That won't be fixed by more files, unless the files are on separate IO subsystems.

Test if it's faster to move the databases by hand rather than over the network (copy to external drive, take to server room). Assuming not cloud, that is.

It prevents performance problems. That's why SQL caches the shells of temp tables, to make the process of creating new temp tables faster.
And you don't truncate them, because they're...

What is this query doing?
Can the process of getting data into the tables be changed so that the delete is not necessary?

It depends completely on the IO subsystem, how fast it can read and write the DB and, if there's a network copy involved, how long that will take.

48...

Ok, the problem is that CASE (SELECT ...) must only return a single column and a single row, because it's part of a scalar expression.
Multiple columns is easy, you...

Please post the entire query.
The code you posted has no subqueries, and hence cannot throw that error. It does however fail to parse with

Msg 102, Level...

They're either table variables or the shells of temp tables which SQL caches so that it doesn't have to do so much work to create new ones.