Final answer:
The major factor in determining how quickly an mRNA is degraded is 5' cap addition.
Step-by-step explanation:
The major factor in determining how quickly an mRNA is degraded is 5' cap addition. While the pre-mRNA is still being synthesized, a 7-methylguanosine cap is added to the 5' end of the growing transcript. This cap protects the mRNA from degradation and helps initiate translation by ribosomes. In contrast, the other options listed, such as polyadenylation, splicing efficiency, and codon usage bias, do not play major roles in mRNA degradation.