执行更多或更复杂的SQL查询，还是使用PHP过滤数据？

I'm building a PHP app which will get a lot of traffic. I have always learned that I should limit the number of SQL queries (right now it's about 15 per visitor). I could limit this to about half, using PHP to filter and sort data, but would this be faster? Alternatively, I could use "JOIN" in the query to reduce the number of queries, but is this really faster than executing multiple queries? Thanks!I'm building a PHP app which will get a lot of