テストをいつ返そうが、返すまいが、生徒にガタガタいわれるスジはない。そんなことはまったくうどうでもいい。採点に時間がかかるのは、悪いことではないでしょ。テストは授業で授業はテスト。
It's none of your students' business when you return their test papers. Teaching is testing, and testing is teaching. Creating a test that takes some time to mark is nothing to be avoided; rather it can be evidence that the test is tapping some productive ability (like extended writing), which of course is a good thing.