The two environments are just "different", separate and distinct. You can do plsql without SQL, you can do SQL (and many times do) without invoking plsql. There is a call overhead to go from SQL to PLSQL (the "hit" is most evident when SQL invokes PLSQL - not so much the other way, when SQL is embedded in PLSQL). Even if this hit is very very small (say 1/1000th of a second) - if you do it enough, it adds up. So, if it can be avoided - it should be.
The session was Using Kanban and Scrum to Increase Your Development Throughput presented by Stew Stryker (not to be confused with Ted Striker) of Dartmouth College (Stew gave me a gallon of Vermont Maple Syrup which exploded in my bag on the flight home, a gift for sharing my hotel room. Thanks Stew! ;)). So here's the example he gave to demonstrate context switching.
Take a list of names and time yourself writing out the first letter of each name, then the second, until you are finished.
Now, same list of names and write them out the way you normally would, left to right.
If the first method was faster, you are a freak of nature.