Learn Dutch: Deze speler is het traagst. - This player is the slowest.

When a superlative adjective is used on its own after a verb (like "is") rather than right in front of a noun, it almost always gets the word het in front of it. This rule applies even if the noun it describes is a de-word like speler.

Since speler is a de-word, could I say "Deze speler is de traagste"?

Yes, you can! If you say de traagste, you are essentially saying "the slowest one" and implying the noun again. When you add that -e ending, the article must match the noun (de speler, so de traagste). However, the fixed form het traagst (with no -e on the end) is the most standard way to say "is the slowest" as a general description.

How is the superlative traagst formed from the base word?

The base adjective is traag (slow). To make the superlative ("slowest"), you simply add -st to the end. Unlike English, which sometimes adds an extra vowel to make it easier to say (slow-est), Dutch just tacks the consonants right on: traag + st = traagst.

Just as a refresher, why do we use Deze for "this" instead of Dit?