I took it as meaning a morning that is relaxed, where you aren't rushing to get anywhere or get things done.
Is there a rule about the letter M becoming double when standing before the letter A?
No, however the consonant doubling generally indicates that the preceding vowel is short.
I have never heard the word "slow" applied to a time of day in English! Lazy? Relaxed? But not slow.
if a slow morning sounds unnatural in the rest of languages but Swedish, let a native explain to us what it really means and let us stop guessing and supposing.