Why do SQL FullText queries slow down when you OR?
Does changing to using ContainsTable help?
It did here Adding more OR searches with CONTAINS Brings Query to Crawl
And the same answerer (Joe Stefanelli) managed to bring about a similar improvement by changing FREETEXT predicates combined with OR to a FREETEXTTABLE here SQL Server full text query across multiple tables - why so slow?
I'd take a look at the execution plan for each of those. I'm guessing you will learn quite a bit from that.
Here is a decent link that will show you how to display the execution plan as well as some tips on interpreting it.