It's definetly not always going to be a problem, but sometimes it does. ORM is always slower than writing custom code, but it might be more than fast enough.
Surely it depends on the custom code in question if it is faster or slower than the similar ORM logic?
You may be able to write faster code, but unless you are Donald Knuth, you can't guarantee that any custom code you write will always be faster than some library.
