Half-life is the time taken by a substance (molecule, drug or other) to lose half of its pharmacological or physiological activity. Used by extension in the field of radioactivity, the half-life, also called radioactive half-life, is the time at which half of the radioactive nuclei of a source have disintegrated. <span>The term half-life is often misinterpreted: two half-lives do not correspond to the full life of the product. The half-life is in fact the median of the life of a product, that is to say the duration below which there remains more than 50% of the product, and beyond which there remains less than 50%.</span>
I read it's has to do with the amount of nucleic acid that must be synthesized. RNAs are tiny compared to DNA, so there is little selection pressure for speed. Also, multiple RNA polymerases can initiate simultaneously, effectively increasing the synthesis rate
