I noticed that when the Olympics were founded only amateur athletes were allowed to compete. 90 years later, professionals were first allowed in the games. I can research all the times for track athletes in the Olympics but they must not have been the best in the world because they're not professional. How good were the professional athletes at the time?
It depends on the sport. In athletics and (especially) swimming there wasn't a huge professional circuit and so the Olympics really were the pinnacle of performance.
In other sports that wasn't the case. Cycling is the clear example: professional cyclists were much better than amateurs both in reputation and in quantitative performance (professional records were better than amateur records).
The whole comparison was complicated after WW2 by both the Soviet block country's approach of having special military units where "amateur" athletes were paid for "military service" and by the US college system where "amateur" athletes were offered scholarships. Other countries had other similar systems.
I'm not familiar enough with the winter Olympics to comment sensibly on them.