Poor performance with lateinit mockks

[ak131554]

I've recently discovered that our tests are needlessly slow. By investigation I found out that the following pattern is slow.

class TestWithLateinitMockk
{
  private lateinit var one: AtomicInteger

  @BeforeEach
  fun setup()
  {
    one = mockk()
    every { one.get() } returns 1
  }

  @Test
  fun testIt()
  {
    Assertions.assertEquals(3, 2 + one.get())
  }
}

In contrary, I have three different Tests:

class BasicTest
{
  private var one: AtomicInteger = AtomicInteger(1)

  @Test
  fun testIt()
  {
    Assertions.assertEquals(3, 2 + one.get())
  }
}

class TestWithLateinit
{
  private lateinit var one: AtomicInteger

  @BeforeEach
  fun setup()
  {
    one = AtomicInteger(1)
  }

  @Test
  fun testIt()
  {
    Assertions.assertEquals(3, 2 + one.get())
  }
}

class TestWithMockk
{
  private var one: AtomicInteger = mockk()

  @BeforeEach
  fun setup()
  {
    every { one.get() } returns 1
  }

  @Test
  fun testIt()
  {
    Assertions.assertEquals(3, 2 + one.get())
  }
}

Each of those three tests run on my machine in low (one or two digits) milliseconds. The lateinit mockk tests needs more than one second for the same Assertion. So it's several orders of magnitude slower. (Please see the working code in my Gitlab Repository, so that you can try it yourself.)

I don't know if this is a bug. If not than maybe someone can explain to me, why lateinit mockks have such an impact on performance. I know that mock(k)ing is a bit slower due to reflection. But I don't understand the difference between initialize on declaration and late initialized mockks.