SQL performance difference JOIN vs UNIONALL

Question

I'm trying to improve a SQL query of mine, where I have to join two tables Post & PostItems. I've been able to figure out that the problem was caused due to an OR clause in my JOIN statement. My original query looks like this:

SELECT P.ID, IT.Total
FROM Post P
JOIN PostItem IT ON IT.PostID = P.ReferencePostID OR IT.PostID = P.ID

It takes this query about 6 minutes to spit out about 5k lines. If I split this query up like so:

SELECT P.ID, IT.Total
FROM Post P
JOIN PostItem IT ON P.ID = IT.PostID
                    
UNION ALL
                    
SELECT P.ID, IT.Total
FROM Post P 
JOIN PostItem IT ON P.ReferencePostID = IT.PostID

It'll spit out the exact same result, but it'll take less than a second. Any idea how this occurs?

For some more context, ID is the primary key of the Post table. PostID is a foreign key to ID. ReferencePostID has no constraint whatsoever.

Thanks in advance!