Read Only Database Performance Question

In a nutshell, I have a stored proc that takes appx 15 minutes when it uses
a read-only database, and about 2 minutes when the database is not
read-only.

Details:

SQL Server 7

The stored proc exists on a writable database on the same server. It stuffs
a subset of about 20000 rows into a temp table and repeatedly updates the
temp table while doing selects and joins on the read only database, for
example,

Select *
into #LocalTempTable
from SAMESERVER.dbo.ReadOnlyDatabaseTable

Update tmp set tmp.SomeColumn = 1
from #LocalTempTable tmp
inner join SAMESERVER.dbo.ReadOnlyDatabaseTable x on
tmp.ID = x.ID
where SomeCriteria = SomeValue, etc, etc.

If I change the read-only database to writable it works fine. There are no
explicit transactions used in the stored proc. On the same server, a
production database exists and is constantly using transactions and temp
tables, but I wouldn't think this is an overall server performance issue
because it handles the same load when the suspect database is not read-only.

Any thoughts will be greatly appreciated. I'm searching all over and can't
seem to find reference to this type of issue.