Coming from discussion #5516, in particular this message: #5516 (comment).

Excerpt:

However, I noticed that when querying a CSS property name that exists unprefixed, it doesn't get cached into the vendorProps map (contrarily to the prefixed names, that get cached).

Therefore, when querying unprefixed names, each time there is one lookup in the vendorProps map then one lookup in the emptyStyle object, whereas we could save the latter (after the cache is populated on first access).

Current code: finalPropName.js

Also, I just noticed that previously the unprefixed names were cached at well, but it is no longer the case since commit 2b5f5d5.