The connection that takes 20min longer has two additional stops (the fast connection is a direct one) but it is still served by TGV or ICE trains, like the direct connection.
The distance between Paris and Strasbourg is >400km, so even the "slow" connection has an average speed of ~200 km/h. The actual regional train connection (TER) takes nearly 5 hours with plenty of stops in between. Slightly faster non-regional but non-TGV connections only exist on lines that are not served by TGVs.