Set queue size = 1 to Copy island right after the desync.
In this case, Copy won't read more data from a "last_written"
container than required, while feeding the desynchronized path.

Sometimes Copy don't get fused into an island and behaves
on its own -- in this case, it reads more data in advance
so the slow (desync) part actually processes some data in-sync
(more than actually required)

force_builders=Custom,Custom Win,Custom Mac
build_gapi_standalone:Linux x64=ade-0.1.1f
build_gapi_standalone:Win64=ade-0.1.1f
build_gapi_standalone:Mac=ade-0.1.1f
build_gapi_standalone:Linux x64 Debug=ade-0.1.1f

Xbuild_image:Custom=centos:7
Xbuildworker:Custom=linux-1
build_gapi_standalone:Custom=ade-0.1.1f

build_image:Custom=ubuntu-openvino-2020.4.0:16.04
build_image:Custom Win=openvino-2020.4.0
build_image:Custom Mac=openvino-2020.4.0

test_modules:Custom=gapi
test_modules:Custom Win=gapi
test_modules:Custom Mac=gapi

buildworker:Custom=linux-1
# disabled due high memory usage: test_opencl:Custom=ON
test_opencl:Custom=OFF
test_bigdata:Custom=1
test_filter:Custom=*

Pull Request Readiness Checklist

See details at https://github.com/opencv/opencv/wiki/How_to_contribute#making-a-good-pull-request

• I agree to contribute to the project under Apache 2 License.
• To the best of my knowledge, the proposed patch is not based on a code under GPL or other license that is incompatible with OpenCV
• The PR is proposed to proper branch
• There is reference to original bug report and related work
• There is accuracy test, performance test and test data in opencv_extra repository, if applicable
  Patch to opencv_extra has the same branch name.
• The feature is well documented and sample code can be built with the project CMake