Sql-server – SQL Join slower than subquery on View with a Function

My database has a view that calls a function. Joining on that view is surprisingly slow. Is there a way to speed up joining to a view like that?

I've included 2 queries which illustrate the issue. They give identical results. The first query takes 6 seconds to run. According to the estimated execution plan, the first query spends 82% of its time in "Nested Loops (Inner Join)". The second query runs much faster, but for more complicated queries I would have to use a join instead of a sub query.

I'm happy to provide more information about my particular system, but I suspect this is a generally applicable issue.

--ptynbr is int
--ptyid is varchar
--vwTest has 8307 rows
--map has 126 rows
--Instrument and Party are plain old tables.

Create function [dbo].[fnTest](@ptynbr int)
returns varchar(40)
as
begin
  declare @result varchar(40);
  select @result = ptyid from dbo.[Party] where ptynbr = @ptynbr;
  return (@result);
end;  
GO

CREATE view [dbo].[vwTest] as 
select dbo.[fnTest](i.ptynbr) PartyName, i.insaddr
from Instrument i
GO

select distinct i.PartyName
into #map
from [vwTest] i


--Takes 6 seconds
select i.insaddr
from [vwTest] i 
join #map m on m.PartyName = i.PartyName

--Takes < 1 second.
 select i.insaddr
from [vwTest] i 
where i.PartyName in (select PartyName from #map)