This is likely because MySQL will use the TC_LOG_MMAP transaction coordinator instead of the MYSQL_BIN_LOG transaction coordinator when TokuDB is enabled which does more fsyncs (because there are now 2 transactional engines enabled within the server).
Vadim has already discovered this phenomenon in his testing in the past and is sort of a known issue. It seems that the MMAP coordinator is not very well optimized.
This is likely because MySQL will use the TC_LOG_MMAP transaction coordinator instead of the MYSQL_BIN_LOG transaction coordinator when TokuDB is enabled which does more fsyncs (because there are now 2 transactional engines enabled within the server).
Vadim has already discovered this phenomenon in his testing in the past and is sort of a known issue. It seems that the MMAP coordinator is not very well optimized.