RuBisCO
The enzyme that captures CO2 and commits it to sugar. It is slow, hugely abundant, and it also reacts with oxygen, which is the flaw that makes photorespiration possible.
Ribulose-1,5-bisphosphate carboxylase/oxygenase is thought to be the most abundant protein on Earth, and it is remarkably slow, fixing only a few molecules of CO2 per second. Plants compensate by making enormous quantities of it, which is part of why leaves demand so much nitrogen.
Its name carries the problem. RuBisCO is both a carboxylase and an oxygenase, so it can attach either CO2 or O2 to its substrate. Attaching oxygen produces a compound the plant must spend energy salvaging, releasing carbon it had already fixed.
Which reaction wins depends on the ratio of the two gases at the enzyme and on temperature. Raising CO2 in the room raises that ratio, and this is the mechanism underneath every claimed benefit of enrichment.