It was “only” 1.4 billion years for multicellular life to evolve: 2-Billion-Year-Old Fossils May Be Earliest Known Multicellular Life
And that may not be the earliest multicellular life. Organisms without hard body parts do not fossilize well.
Still, there was a lot to evolve in that 1.4 billion years. First, directed protein synthesis needed to evolve. As the Comment notes, the first organisms came from proteins formed abiotically. Those proteins catalyzed the formation of new proteins from amino acids. However, that is not the same as using DNA to code for proteins (directed protein synthesis). It took time to evolve the ribosomes to catalyze protein synthesis, then more time to evolve the DNA code and the biochemical steps to transcribe DNA to mRNA.
Second, standard and efficient biochemical pathways were evolving. The Krebs cycle, lipid biosynthesis, synthesis of amino acids, etc. All those pathways called “intermediary metabolism”.
Third, eukaryotes were evolving. That is, cells with a nucleus.Also during this time the endosymbiosis of the prokaryotes that became mitochondria and chloroplasts was happening.
Only when these different parts were evolved, or had evolved close to their current conditions, was the evolution of multicellular organisms as we know them possible.
IOW, a lot of “background” evolution was happening at the biochemical level, and this took time.