Associative Array potential performance pitfall?

[Adnan]

In my machine the following D code compiled with release flag and 
LDC performs over 230ms while the similar Go code performs under 
120ms.

string smallestRepr(const string arg) {
	import std.format : format;

	const repeated = format!"%s%s"(arg, arg);
	string result;
	result.reserve(arg.length);
	result = arg;
	foreach (i; 1 .. arg.length) {
		const slice = repeated[i .. i + arg.length];
		if (slice < result)
			result = slice;
	}
	return result;
}

unittest {
	assert("cba".smallestRepr() == "acb");
}

void main(const string[] args) {
	string[][const string] wordTable;
	import std.stdio : writeln, lines, File;

	foreach (string word; File(args[1]).lines()) {
		word = word[0 .. $ - 1]; // strip the newline character
		const string key = word.smallestRepr();
		if (key in wordTable)
			wordTable[key] ~= word;
		else
			wordTable[key] = [word];
	}

	foreach (key; wordTable.keys) {
		if (wordTable[key].length == 4) {
			writeln(wordTable[key]);
			break;
		}
	}
}

---

package main

import (
         "bufio"
         "fmt"
         "os"
)

func canonicalize(s string) string {
         c := s + s[:len(s)-1]
         best := s
         for i := 1; i < len(s); i++ {
                 if c[i:i+len(s)] < best {
                         best = c[i : i+len(s)]
                 }
         }
         return best
}

func main() {
         seen := make(map[string][]string)

         inputFile, err := os.Open(os.Args[1])
         defer inputFile.Close()

         if err != nil {
                 os.Exit(1)
         }

         scanner := bufio.NewScanner(inputFile)

         for scanner.Scan() {
                 word := scanner.Text()
                 n := canonicalize(word)
                 seen[n] = append(seen[n], word)

         }

         for _, words := range seen {
                 if len(words) == 4 {
                         fmt.Printf("%v\n", words)
                         break
                 }

         }
}

The input file : 
https://raw.githubusercontent.com/dolph/dictionary/master/enable1.txthttps://raw.githubusercontent.com/dolph/dictionary/master/enable1.txt

Problem description: 
https://www.reddit.com/r/dailyprogrammer/comments/ffxabb/20200309_challenge_383_easy_necklace_matching

Where am I losing performance?