From the Way Too Much Time on my Hands basket, I was thinking of the ways the Beatles music changed over the years, but in particular that period from 62 to 65 when they went from Please Please Me to Rubber Soul.
An obvious point is that they went from being a rock and roll band pumping out uptempo pop/rock tunes intended for live performances to mature songwriters composing in a variety of styles.
To see if I could quantify this I found a database with the beats per minute (BPM) of every Beatle song. Now BPMs are an imperfect measure as some songs with relatively low BPMs can sound uptempo and vice versa, but they do give at least some indication of the speeds of songs the Beatles were performing.
Ignoring the singles for now I looked at the average BPMs for their studio albums up to Rubber Soul. The results are reasonably stark:
Please Please Me 132
With the Beatles 126
A Hard Days Night 121
Beatles for Sale. 116
Rubber Soul. 110
The lads basically dropped 5 BPMs from one album to the next up to Help. Rubber Soul and Help had basically the same average. I'd read somewhere a while ago that a marked change was that while the Beatle tempos slowed down, they increased the "internal" tempo. Meaning they used more 16th notes and the like within the slower tempo to provide the pace of the song.
For drummers in particular who have played lots of Beatle songs from their different eras none of this would be actually surprising, But what can I say, we guitarists need everything spelled out for us.
For those of a statistical mind I didn't do any robustness tests (I'm not that bored) So whether these meet standard tests I don't know. As the results fits my preconceived notion I'll just accept them as they are.