As individual words, "ist" and "isst" are pronounced completely identically.
In the sentence you mentioned, though, "is" is not stressed but "eats" can be -- "Er ist lángsam" vs. "Er ísst lángsam" (but could also be "Er isst lángsam" - no difference in pronunciation from "Er ist lángsam" - only context can tell).
Yes, I misread "isst" as "ist", and put "He is slow". Duolinbgo "corrected" me to "He eats slow". That is just wrong in English. The word is an adverb here, and the adverb in English is "slowly". ("Slow" is an adjective).
how do you spell, or say, 'SLOW' in German?
Same word as "slowly", in other words.
Nearly all German adjectives can be used as adverbs in this way, just by taking the stem of the adjective (without any ending).
For example, Er tanzt gut. "He dances well."
Many Germans make mistakes in English because of this and say things such as "He dances good".
isst and esst shout have the same meaning
They do, in the sense that "am" and "is" have the same meaning -- they're both forms of the same verb.
But you can't say "I is" or "he am" -- and you can't say er esst or ihr isst. You have to use the verb form that matches the subject.