MySQL replication much slower on MySQL 5.7 compared to MySQL 5.5
I appear to have found the solution. Simply put I set MySQL 5.5 to also use ROW based Replication, using the following process:

SELECT @@GLOBAL.binlog_format;
STOP SLAVE;
FLUSH TABLES WITH READ LOCK;
FLUSH LOGS;
SET GLOBAL binlog_format = 'row';
SET SESSION binlog_format = 'row';
FLUSH LOGS;
UNLOCK TABLES;  
START SLAVE;
SELECT @@GLOBAL.binlog_format;


After this, I ran the large query on MySQL 5.5 (ran instantly), and when it replicated to MySQL 5.7, it also ran instantly. 

So there appears to be some problem with MySQL 5.7 processing statement based replication queries from a MySQL 5.5 server.

. . . But the plot thickens. Although the MySQL 5.5 database claimed to be using ROW Based Replication:

SELECT @@GLOBAL.binlog_format; = ROW

and the fact that this appears to have helped with replication to MySQL 5.7, when I grabbed the latest binary logs (an hour later or about 10 binary logs later) and ran mysqlbinlog against them, I was surprised to see that they were still populated with STATEMENTs. It was only after a full restart of the database that the binary logs started producing the new ROW based format ! ! !