Some information online indicates that the non high speed train takes about 20 mins more than the high speed train on that route. It does not seem a huge time difference
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.
One took maybe 6 hours. The other 12+ or some such. The 12+ hour took almost the same route, but stopped at every. single. town.
Woe to the person wanting to go from Ottawa to Toronto, and buying the wrong ticket. This is pre-Internet so research was less common and easy, and if you have no idea it could matter...
I recall this being named the "milk run".