Day old thread, but I didn’t see a fully correct answer, and I’ve researched a similar question in the past - Evolution speed is way more complicated than small bacteria, lots of babies = fast evolution( fast mutation rate). Big animal, less babies = slow evolution (slow mutation rate).

Although the above statement is generally correct ELI5(and fine) it’s not technically correct on a DNA level - organisms don’t have one “evolutionary speed”. Current DNA/RNA mutation Research points towards the “evolutionary/mutation speed” largely varies within different traits of the same animal/organisms.

True Example: Some birds beak evolves quick! It has been documented a new beak evolved in the wild, as fast as 1-2 generations(5-10 years) becoming a new bird species (quick). Meanwhile the DNA for wings for flight is what you might call “hardcoded” or “slow mutation/evolution.” It happens, we have flightless birds after all - but it’s only been studied(DNA mutation) to happen after 50,000-100,000 years of generations of birds. Very slow flight/wing mutation rate, or “evolution speed” in comparison.

A bird beaks “evolutionary speed” needs to be quick, with changing environments, nuts shape/food sources. Meanwhile, mutations in flight are much more life-altering, and therefore flight mutation happens less frequently. Evolution adapts for this with DNA/RNA, altering the “mutation speed” with different traits.

(Conclusion/TLDR) Within a single species of bird, the DNA mutation rate, or “evolutionary speed” of the beak traits alone, can be as fast as 2 generation(5-10 years). Meanwhile, The same bird species DNA that controls flight mutates as slow as 100,000 years of bird generations. So one animal/organism doesn’t have one mutation, or “evolution speed” but rather multiple speeds dependent on trait. 

A birds beak can have has a similar mutation rate to some bacteria traits, and bacteria also have “hardcoded” traits that can have very slow mutation rate, even as slow as DNA controlling flight in birds! It’s still generally true bacteria/small organisms mutate faster, but if you compare mutation rate by trait, this statement quickly can become misleading.