Not opaque - and buffers are really large. If they do not get mipmapped the GPU cache will hate us.
With additional and enabled traces I get the following logs with 9 applications started:

ApplicationScreenshotProvider::requestPixmap (this=0x47bd58f8, id=friends-app)
ApplicationScreenshotProvider - working with size 768 x 1222
ApplicationScreenshotProvider::requestPixmap (this=0x47bd58f8, id=com.ubuntu.sudoku_sudoku_1.0.142)
ApplicationScreenshotProvider - working with size 768 x 1222
ApplicationScreenshotProvider::requestPixmap (this=0x47bd58f8, id=gallery-app)
ApplicationScreenshotProvider - working with size 768 x 1280
ApplicationScreenshotProvider::requestPixmap (this=0x47bd58f8, id=com.ubuntu.calendar_calendar_0.4.182)
ApplicationScreenshotProvider - working with size 768 x 1222
ApplicationScreenshotProvider::requestPixmap (this=0x47bd58f8, id=com.ubuntu.calculator_calculator_0.1.3.206)
ApplicationScreenshotProvider - working with size 768 x 1222
ApplicationScreenshotProvider::requestPixmap (this=0x47bd58f8, id=camera-app)
ApplicationScreenshotProvider - working with size 768 x 1280
ApplicationScreenshotProvider::requestPixmap (this=0x47bd58f8, id=webbrowser-app)
ApplicationScreenshotProvider - working with size 768 x 1222
ApplicationScreenshotProvider::requestPixmap (this=0x47bd58f8, id=address-book-app)
ApplicationScreenshotProvider - working with size 768 x 1222
ApplicationScreenshotProvider::requestPixmap (this=0x47bd58f8, id=dialer-app)
ApplicationScreenshotProvider - working with size 768 x 1222

Those are the screenshots requested from the scene that shows the application  thumbnails.

When I click on application (here dialer-app) I get:
ApplicationScreenshotProvider::requestPixmap (this=0x47bd58f8, id=dialer-app)
ApplicationScreenshotProvider - working with size 768 x 1222
ApplicationScreenshotProvider::requestPixmap (this=0x47bd58f8, id=gallery-app)
ApplicationScreenshotProvider - working with size 768 x 1280
ApplicationScreenshotProvider::requestPixmap (this=0x47bd58f8, id=com.ubuntu.calendar_calendar_0.4.182)
ApplicationScreenshotProvider - working with size 768 x 1222
ApplicationScreenshotProvider::requestPixmap (this=0x47bd58f8, id=com.ubuntu.calculator_calculator_0.1.3.206)
ApplicationScreenshotProvider - working with size 768 x 1222
ApplicationScreenshotProvider::requestPixmap (this=0x47bd58f8, id=com.ubuntu.sudoku_sudoku_1.0.142)
ApplicationScreenshotProvider - working with size 768 x 1222
ApplicationScreenshotProvider::requestPixmap (this=0x47bd58f8, id=camera-app)
ApplicationScreenshotProvider - working with size 768 x 1280
ApplicationScreenshotProvider::requestPixmap (this=0x47bd58f8, id=webbrowser-app)
ApplicationScreenshotProvider - working with size 768 x 1222
ApplicationScreenshotProvider::requestPixmap (this=0x47bd58f8, id=address-book-app)
ApplicationScreenshotProvider - working with size 768 x 1222
ApplicationScreenshotProvider::requestPixmap (this=0x47bd58f8, id=dialer-app)
ApplicationScreenshotProvider - working with size 768 x 1222
ApplicationScreenshotProvider::requestPixmap (this=0x47bd58f8, id=friends-app)
ApplicationScreenshotProvider - working with size 768 x 1222
ApplicationScreenshotProvider::requestPixmap (this=0x47bd58f8, id=dialer-app)
ApplicationScreenshotProvider - working with size 768 x 1222
ApplicationScreenshotProvider::requestPixmap (this=0x47bd58f8, id=dialer-app)
ApplicationScreenshotProvider - working with size 768 x 1222
ApplicationScreenshotProvider::requestPixmap (this=0x47bd58f8, id=friends-app)
ApplicationScreenshotProvider - working with size 768 x 1222

Note: three times dialer-app!
When I swipe left-right to switch from dialer to browser:

ApplicationScreenshotProvider::requestPixmap (this=0x47bd58f8, id=webbrowser-app)
ApplicationScreenshotProvider - working with size 768 x 1222
ApplicationScreenshotProvider::requestPixmap (this=0x47bd58f8, id=dialer-app)
ApplicationScreenshotProvider - working with size 768 x 1222

.. and when the application switch ends:
ApplicationScreenshotProvider::requestPixmap (this=0x47bd58f8, id=webbrowser-app)
ApplicationScreenshotProvider - working with size 768 x 1222
ApplicationScreenshotProvider::requestPixmap (this=0x47bd58f8, id=dialer-app)
ApplicationScreenshotProvider - working with size 768 x 1222
ApplicationScreenshotProvider::requestPixmap (this=0x47bd58f8, id=address-book-app)
ApplicationScreenshotProvider - working with size 768 x 1222
ApplicationScreenshotProvider::requestPixmap (this=0x47bd58f8, id=friends-app)
ApplicationScreenshotProvider - working with size 768 x 1222
ApplicationScreenshotProvider::requestPixmap (this=0x47bd58f8, id=gallery-app)
ApplicationScreenshotProvider - working with size 768 x 1280
ApplicationScreenshotProvider::requestPixmap (this=0x47bd58f8, id=com.ubuntu.calendar_calendar_0.4.182)
ApplicationScreenshotProvider - working with size 768 x 1222
ApplicationScreenshotProvider::requestPixmap (this=0x47bd58f8, id=com.ubuntu.calculator_calculator_0.1.3.206)
ApplicationScreenshotProvider - working with size 768 x 1222
ApplicationScreenshotProvider::requestPixmap (this=0x47bd58f8, id=com.ubuntu.sudoku_sudoku_1.0.142)
ApplicationScreenshotProvider - working with size 768 x 1222
ApplicationScreenshotProvider::requestPixmap (this=0x47bd58f8, id=camera-app)
ApplicationScreenshotProvider - working with size 768 x 1280


Sorry for taking so long, I was suspecting a misbehaviour in mir so looked at the other end of things, working upwards since I found nothing.

Now I will try to find the place that triggers those loads - I guess we could do some caching on this or a different level and especially use the image scaling.