If you called someone "langsam", it would need a context to actually mean "slow" (as in stupid or unpreferrable). Out of context, it does not have as strongly as in English the meaning of "slow"= stupid (yet it can mean that in a certain context).
"Wir haben einen neuen Mitarbeiter. Er ist so langsam." = We have a new employee. He is so slow." - Here, slow could literally mean slow in terms of walking, working, etc. not necessarily slow with regards to "not very intelligent".
What is the composition of the word langsam? I mean, is it just an addition of lang with sam? And what does Sam mean?