This seems to me an unexpected mixture of two sayings:
"Lento pero seguro", which means the person is slow for doing some things, but you can still count on them being done... someday.
"Tarde, pero sin sueño" (should rather be "llegué tarde, pero sin sueño", but the verb is ellided), which is a cheeky way to say you're late to your appointment, but at least you won't fall asleep. Mostly said on morning appointments or meetings, when it's assumed you're late because you overslept.
Mexicans use both sayings quite often, but not the mixture.