A subquery could run in O(k) time which is very fast. In fact, it could be merely a mathematical operation (e.g., 1+1). But it could run in O(n!) which is very slow. A subquery could be a join.

Joins can be one of three types: nested loops, merge joins and hash joins. Nested loops are appropriate for small joins. Merge joins will be chosen by the database optimizer when the tables are almost in the correct order for the join. Merge join is faster than hash joins and takes less space in tempdb. Hash joins are chosen when the tables are essentially randomized compared to their intended result. Hash joins tend to be the slowest and take up the most space in tempdb. The underlying type of join is vieweable in the execution plan. A join operation is a supported operation of SQL Server. The operation itself, I don't think, could include a subquery. A transaction could encompass a join and a subquery.