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
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. "
I looked up English "boring " and this dictionary said Dutch "saai", and it came back from Dutch to English (saai = boring). http://webtranslation.paralink.com/translator/default.asp I looked up German "langsam" and this dictionary said Dutch "vertragen". I tried Dutch to English ("vertragen" came back as "slow down".) http://webtranslation.paralink.com/translator/default.asp (Hmm, the same link came up, so I guess you must enter the languages and words yourself each time.) When I went from Dutch to German it put the same "langsaam" to "langsaam", I don't know if this is right at all. I need a better dictionary.
"Boring (langweilig)" may be interpreted from "slow (langsam)", but it is not a directly-used synonym. Or also be meant in a humorous way. But, please do not try to equate these expressions, because this interpretation of "slow (langsam)" is a habit or it might be a part of that person's diction.
As someone who speaks german as a mother tongue, I can say that "langsam" doesn't mean boring, but slow. Boring would be "langweilig" and if you would translate "langweilig" split into "lang" and "weilig", it would be "a long while" or something like this. So langsam/langzaam don't have any connection to boring ;)