Merging coverage data using lcov -a with branch coverage enabled is so slow that it makes it unusable.
Please, try to optimize this operation.
I have ~650 partial files which have to be merged, but merging every two of them takes 30-45 minutes. Even using good 8 cores machine it will take ~2 weeks to merge all of them. Moreover, these files were narrowed down to cut unnecessary data. Merging the same files with branch coverage disabled takes ~10 seconds.