Translation:He swims fast and runs slow.
他游泳游的很快 this sentence is like "He swims, (swim) very fast. 游泳，跑步 are seperable verbs. You need to duplicate the main verb again （which are 游，跑 here） before describe how the subject do it. So you may say 我游得很快。or 我“游泳”游的很快。You may watch this video for further information. https://www.youtube.com/watch?v=ggPbyZN96ss Start from 8:40
The adverb "slow" do exist, however, it appears it is mainly used in the US. Duolingo seems to adopt American English.
https://dictionary.cambridge.org/dictionary/english/slow mainly us: He drives too slow!
American English is losing its adverbs (in a descriptive sense), but they're still prescribed. I would mark this wrong if a student gave it to me. Why that dictionary has added it at the bottom is a mystery - it's really not standard (American) English, though it does reflect common usage in some dialects.
If you're going to respond entirely in Chinese, please make sure you're using the correct characters. You have some errors, and it makes it so that anyone who needs to look up anything you wrote probably won't be able to figure out what you mean. 叫 is "to call." 教 is "to teach." I haven't heard this grammatical construction related to time spent doing something myself, but I have heard this construction in something like 我游不泳了, which means "I can't swim." I'm not proficient with that construction, though, so I would normally say this differently. Whether you can use it or not depends on whether or not the second word in the verb is really a verb or a noun independently. I don't know if it can actually be used with 游泳 or not.
I was confused by this too. Check out this link. It should provide some guidance. Look under "The common usages of separable words" , specifically section 3.
The link provided above by Michaelingua (https://www.digmandarin.com/chinese-separable-words.html) is very helpful.
Either watch the video in the first post, or this is very helpful too, explains it all: https://resources.allsetlearning.com/chinese/grammar/Degree_complement#Degree_Complements_with_Objects.