Slow scalar function that used to be fast
You are requerying the Account_Profiles table for every single row in OrderItems.  If OrderItems has 50,000 rows then you are going to hit Account_Profiles 50,000 times.  If OrderItems grows larger you will query it more times.  If Account_Profiles grows larger then each of those times will take longer.  You will see a large difference by inlining the concatenation. (making assumptions on the join column names here)

Select Firstname + ' ' + Lastname FullName --other data here
FROM OrderItems oi INNER JOIN Account_Profiles ap
ON oi.userid=ap.userid