The oldest of the Vibroplex Originals had thinner springs which implies the springs were weaker and thus slower.
K4MSG says in his eham review of Alberto Frattini's J-36 semiautomatic key:
Much lower minimum speed range than my Lionel J-36. I did a rough measurement and it’s in the neighborhood of 15 wpm minimum with a “high end” capability well above 35 wpm. This is due to the much-thinner mainspring, i.e., 0.0123” on the Frattini J36 as compared to 0.0165” on my 1944 Lionel J-36.
KC4TXR observes Vibroplex bugs made since around January 2001 have a thinner mainspring than those made in 1999, he replaced the mainspring in his 1999 Vibroplex Original with Blue Racer finish and reports that with the weights he has and the longer pendulum length used in all Originals he achieved a slow speed of 16 to 18 wpm with the thinner mainspring used after 1999.
I don't know what Vibroplex Originals use now. The thinness and length of the mainspring make for slower dots as does more weight on the vibrator rod, or any devices that add length to the vibrator rod such as Franklin Geisel, W7IS produced under the "extendadot" name as shown here
https://youtu.be/bSexPcF90VA?si=ACt7tKUAW-ZoP1iK
Perhaps a search of the archives of morsecode and cw email lists on
qsl.net will show historical discussions on mainspring thinness measurements of various Vibroplex semiautomatic keys throughout the years.
73
DR