The lap times says it is more then that. Something like 1s. Just going from 2000 to 2001 there was a massive change to lap times even though the minimum weight rules were not changed. There was some other changes going from 2000 to 2001 (limit rear wing aero etc.). But still they went around 1 to 3 seconds quicker.
https://www.formula1.com/en/results.html/2000/fastest-laps.html
https://www.formula1.com/en/results.html/2001/fastest-laps.html
There was no TC allowed in Australia, Malaysia, Brazil and San Marino and you can clearly see, when comparing qualifying times, they still improved 2-4 seconds per lap compared to 2000.
It was the massive increase in tyre grip thanks to tyre war, which decreased lap times so significantly. Exactly the same thing happened between 1996 and 1997, when Bridgestone entered the sport.
Though the engines just changed to V10s in 2000 so those probably still kept going forwards.
Huh? Renault was running their V10 since 1989, Mercedes since 1994 and Ferrari since 1996.
A better comparison is going from 2007 to 2008 when they lost somewhere around 1 second even though the only rule change in the cars themselves that was changed was the TC ban and standard ECU. Every other part should have gone forward so the loss from TC probably was more then the ~1s.
https://www.formula1.com/en/results.html/2007/fastest-laps.html
https://www.formula1.com/en/results.html/2008/fastest-laps.html
Comparing fastest laps is silly, but hey, let's go for it:
2007 Australia 1:25.235 2008 Australia 1:27.418
2007 Malaysia 1:36.701 2008 Malaysia 1:35.366
2007 Bahrain 1:34.067 2008 Bahrain 1:33.193
2007 Spain 1:22.680 2008 Spain 1:21.670
2007 Turkey 1:27.295 2008 Turkey 1:26.506
Somehow I see they actually have gained 1 second, but hey.
I couldn't find any solid source, but one post at F1Technical mentions that Steve Matchett, former F1 mechanic, said TC gives 1-3 tenths per lap and better consistency during long runs, that's about it.