1-wire behaviour on Bookworm

After getting another Pi 5, I've moved a Pi 4B running Bookworm in to the role taken by a Pi 3B running Bullseye, which is monitoring 6 ds18b20 sensors, configured as 2 1-wire buses with 3 sensors on each.
With the Pi 3B, I could either read the sensors sequentially taking about 0.9s each, or using threads simultaneously in 1.0s. The Pi 4B with Bookworm reads the sensors sequentially a shade faster at just over 0.8s, but when reading simultaneously, returns 2 after 0.8s, another 2 after 1.6s and the last two at 2.4s.
Anyone have an explanation for this?
It's not a problem as I changed some time ago from performing one simultaneous read every 5 minutes to reading sequentially every minute and averaging over 15 minutes.
---druck