Is anyone able to explain really poor performance when using 'If Exists'?

Question

Hello all.  We've recently had a performance spike when using the 'if exists' construct, which is a construct that we use through much of our code.  The problem is, it appears illogical since if can be removed via a tiny modification that does
not change the core code.
I can demonstrate  
This is the (simplified) format of the base original code.  It's purpose is to identify when a column value has changed comparing to a main table and a complex view
select 1 from MainTable m
inner join ComplexView v on m.col2 = v.col2
where m.col3 <> v.col3
This is doing a table scan, however the table only has 17000 rows while the view only has 7000 rows.  The sql executes in approximately 3 seconds.
However if we add the 'If Exists' construct around the original query, like such:
if exists (
select 1 from MainTable m
inner join ComplexView v on m.col2 = v.col2
where m.col3 <> v.col3
print 1
The sql now takes over 2 minutes to run.  Note that the core sql is unchanged.  All I have done is wrap it with 'If Exists'
I can't fathom why the if exists construct is taking so much longer, especially since the code code is unchanged, however more importantly I would like to understand why since we commonly use this type of syntax.
Any advice would be greatly appreciated

Answer 1

OK, that's interesting.  Adding the top 1 clause greatly affects the performance (in a bad way).
The original query (as below) still runs in a few seconds.
select 1 from MainTable m
inner join ComplexView v on m.col2 = v.col2
where m.col3 <> v.col3
The 'Top 1' query (as below) takes almost 2 minutes however.  It's exactly the same query, but with 'top 1' added to it.
select top 1 1 from MainTable m
inner join ComplexView v on m.col2 = v.col2
where m.col3 <> v.col3
I suspect that the top 1 is performing a very similar operation to the exists, in that it is 'supposed' to exit as soon as it finds a single row that satisfies it's results.
It's still not really any closer to making me understand what is causing the issue however.