From the tips & notes of the Adjectives Basics skill:
"Dutch adjectives are usually made by adding an -e to the end of the word. Klein becomes kleine, lang becomes lange. It is placed between the article and the noun: de lange kat (the long cat), het kleine meisje (the little girl).
However, things get a bit complicated with the article een. The -e rule is then only still true for nouns which have the definite article de."
Schildpad is a de-word, so the adjective gets an -e at the end. This opens the last syllable of "langzaam", so the aa is turned into a according to the Dutch spelling rules.
Ahh, I think you are confused by the construction "Het is ... ". This construction can be used for both de-words and het-words. Compare this to the English "It's a boy!", even though nobody would refer to a boy as "it".
"Het is een man. De man loopt." - "It is a man. The man walks."
"Het is een kind. Het kind loopt." - "It is a child. The child walks."
You could say "De schildpad is langzaam." or "The turtle is slow. "When the adjective is on the other side of the verb, it is being used as a predicate adjective. It does refer to the noun which is the subject, but it is not right before the noun and does not add the -e at the end of the predicate adjective. You could say "De schildpad is een dier. Het is langzaam." which is "The turtle is an animal. It is slow. "
At the beginning of a ssentence by itself before the verb "het" means "it". When it is in front of a noun, it means "the" and in some cases English may use "this" or "that" when Dutch uses "the". https://dictionary.reverso.net/dutch-english/het https://dictionary.reverso.net/english-dutch/that https://dictionary.reverso.net/english-dutch/this