A Guess! Your Battery is bad. Get a load test, most OE batteries do not last
much past 5 years.The Speedo is electronic so it will jump under these
conditions.

I have a 5 yo Malibu, V6, 98k miles.

The car is garaged and the temp is  mild (like 50 deg at night)

This morning, whe I tried to start it, I got half a crank, then
nothing. Thought my battery was dead, but the radio was still running
(however the headlights were dim) I tried again and then I got one full
(but slow) crank.

I tried about 8 times, I waited about 10 seconds between each try and
every time it would crank a little more before dying. The car finally
started, but the started sounds slower than usual.

During all that, after each crank, the spedometer jumped to 60 mph
(which I guess is a relevant code from the ecu)

I drove my car to work, and once it was warm it starts easier, the
started sounds slow though.

Does anyone know what the 60mph means? Or how to get more info out of
the computer? It does not sound like my battery is dead (even though I
have no experience with a dead car battery) but I'm guessing that the
more you try starting, the less charge there should be...

Any thought?

dead battery.