Transmission rate of vintage Petzval type lenses

As a rule of thumb, loss is generally assumed to be about 4% per glass to air interface in uncoated lenses. So you will have a transmission of 0.96 per interface.

A true petzval lens has six glass to air interfaces and two cemented glass-glass interfaces. Likely, the cemented interfaces do not make a major contribution to loss unless the cement hasd become visibly defective.

So if we take the transmission of 0.96 to the power of six, we get appx. 0.78 - which means an overall light loss of around a half stop. This would often not matter much depending on the film/sensor used, even more if adapted to a camera with internal metering. An externally metered camera should be compensated a half stop slower.

If the actual SPEED of lenses is the question here, that doesn't only involve transmission, but also practical apertures.

A lens from a time where Petzval lenses were state of the art will be designed for a large format camera of some description. Large format lenses, due to the longer focal lengths needed combined with the requirement of illuminating a large imaging circle, generally are not designed with very fast apertures - though the history information given in marketing literature for some new petzval lenses suggest that f/3.6 at 160mm focal length was possible, which is remarkably fast for a large format lens - though the focal length of 160mm suggests that a plate size was used that, while still large format, was not huge - otherwise 160mm would be too short for portraiture use. Actually, a f/3.6 lens, combined with the glass losses I discussed in another answer, would be about as "bright" as a typical modern entry level DSLR/DSLM zoom lens.