Too slow build in compare with Qt/qmake

[Dushistov]

If I change one line of C++ code in my project and then run
ninja project_apk it takes 39s, gradle output looks like this:

❯ grep Took /tmp/apk-build.log  | grep -v 'Took 0'
:mergeDebugAssets (Thread[Execution worker for ':' Thread 30,5,main]) completed. Took 3.964 secs.
:mergeDebugJniLibFolders (Thread[Execution worker for ':' Thread 30,5,main]) completed. Took 15.387 secs.
:mergeDebugNativeLibs (Thread[Execution worker for ':' Thread 30,5,main]) completed. Took 15.046 secs.
:stripDebugDebugSymbols (Thread[Execution worker for ':' Thread 30,5,main]) completed. Took 3.243 secs.

as you can see the most time is spend during mergeDebug* tasks.
In my ./project/project-arm64-v8a/libs/arm64-v8a/ there are 91 libraries from Qt,
one my library and libc++_shared.

So the most of the time I supposed is spend during handling of Qt libraries.

The most strange part is that Qt libraries recreated every build.
I disable this:

@@ -303,8 +306,8 @@ macro(add_qt_android_apk TARGET SOURCE_TARGET)
         DEPENDS ${SOURCE_TARGET}
         ${QT_ANDROID_PRE_COMMANDS}
         # it seems that recompiled libraries are not copied if we don't remove them first
-        COMMAND ${CMAKE_COMMAND} -E remove_directory ${QT_ANDROID_APP_BINARY_DIR}/libs/${ANDROID_ABI}
-        COMMAND ${CMAKE_COMMAND} -E make_directory ${QT_ANDROID_APP_BINARY_DIR}/libs/${ANDROID_ABI}
+#        COMMAND ${CMAKE_COMMAND} -E remove_directory ${QT_ANDROID_APP_BINARY_DIR}/libs/${ANDROID_ABI}
+#        COMMAND ${CMAKE_COMMAND} -E make_directory ${QT_ANDROID_APP_BINARY_DIR}/libs/${ANDROID_ABI}
         COMMAND ${CMAKE_COMMAND} -E copy ${QT_ANDROID_APP_PATH} ${QT_ANDROID_APP_BINARY_DIR}/libs/${ANDROID_ABI}
         COMMAND ${QT_ANDROID_QT_ROOT}/bin/androiddeployqt
         --verbose

and build time changed to (editing the same file and rebuild project_apk):

❯ grep Took /tmp/apk-build2.log  | grep -v 'Took 0'
:mergeDebugAssets (Thread[Execution worker for ':' Thread 13,5,main]) completed. Took 3.985 secs.
:mergeDebugJniLibFolders (Thread[Execution worker for ':' Thread 13,5,main]) completed. Took 3.987 secs.
:mergeDebugNativeLibs (Thread[Execution worker for ':' Thread 13,5,main]) completed. Took 3.994 secs.
:javaPreCompileDebug (Thread[Execution worker for ':' Thread 12,5,main]) completed. Took 12.674 secs.

So total gradle time changed from 39s to 14s.

I read comment:

it seems that recompiled libraries are not copied if we don't remove them first

But my libproject_arm64-v8a.so updated even after commenting this lines,
while "Qt libraries" not updated, and this is good.