For some reason the Document class triggers a time consuming garbage collect with gcc 5.4.0 (only one I've tried).

This was seen by compiling precompiled versions of VideoTrack.cpp with and without the Document class using the -ftime-report flag to gcc.

When Document is included the compilation takes 2.2 seconds instead of 2.0 seconds otherwise. -ftime-report says that 160ms of those 200ms is "garbage collect".

If gcc is called with --param ggc-min-expand=200 or with --param ggc-min-heapsize=256000 the gc is avoided and the compilation is 160 milliseconds faster. 

Setting a heapsize seems harmless but might of course result in a slightly higher memory usage.

Since clang is the default compiler I'm not going to do anything else but I wanted to document it before I forgot about it.