On db.com, if you select "all but ICE" as means of transport this does remove most HS services, certainly TGVs in France.
You could also use the European Rail Timetable (
http://www.europeanrailtimetable.eu/ ) to work out manually an itinerary without HS.
db.com is Deutsche Bank. You need to search for bahn.de - which goes straight to the DB/Hafas search site, which (essentially) covers all of Europe.
However, heartily agree with the 'all without ICE' option on bahn.de/Hafas, and the European Rail Timetable
As to the original question, it is not an easy one to answer, and I fear it really is one of those situations where there is no substitute for experience. It might be worth specifying a few specific aims then those of us with long standing expereience (yes I'm an ancient 55 in a few days time so I have about 50 years of said experience!) can offer some more views and advice.
There really are many variables. For a start, some German 'traditional trains' use high speed lines - many with noise barriers etc, so you might get just as 'bad' an experience on that as on a high speed train, whereas conversely on a double deck French TGV you can get above noise barrier level by travelling on the top deck, where incidentally the sensation of high speed is lessened.
Generally speaking, outside France it is possible to travel between main places with something other than a high speed train quite a few times per day, though as has been mentioned, it takes time. But once again there are variables on different routes.
In France, there may be just one or two chances a day to do a route without high speed, especially as long distances are involved. The very reason France introduced TGVs is because it took most of a day to travel , say, Paris - Marseille on a traditional train. Having said that ordinary trains have crept back onto some French traditional main lines. For example the classic Paris - Lyon line long distance service was (deliberately) cut to a skeleton one train a day left after TGV was introduced on the 1980s as high speed was the 'only way in future'
Paris - Strasbourg was 'skeletonised' similarly, and is difficult to do non TGV at present - however this is one route where the bahn.de / Hafas planner 'all without ICE' feature comes into its own.
I've just checked Paris - Strasbourg randomly and got a itinerary along the classic route using three separate regional trains (changing at Vitry Le Francois and Metz) one of which is definitely loco hauled Corail, and another of the three might be too.
I tried Paris - Berlin and got one itinerary that can be completed in one day - but that chose the IC Bus from Antwerpen to Dusseldorf, rather killing the point!
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