I remember Apple got sued for purposely slowing things down; wouldn't that mean that they wouldn't do the same for recent OS updates?
What Apple did was to really slowing down devices over updates, but over battery degradation, to avoid phones shutting down randomly or not being able to deliver peak voltage to the SoC when needed. - This behaviour still exists, but is optional to users and can be turned off in Settings once it takes effect.
12.4.6 is not just a . release upgrade but a .x. upgrade. It is not meant to come with new features that could take up CPU cycles; It's a bug fix and security release; You should notice close to 0 difference. Upgrade recommended