diff --git a/src/lib/__tests__/githubYearInReview.test.ts b/src/lib/__tests__/githubYearInReview.test.ts
index f89b1ac..55216c5 100644
--- a/src/lib/__tests__/githubYearInReview.test.ts
+++ b/src/lib/__tests__/githubYearInReview.test.ts
@@ -408,6 +408,87 @@ describe("fetchYearInReviewData additional coverage", () => {
         }
     });
 
+
+
+
+    it("handles missing repository data in mergeTopRepository", async () => {
+        mockFetch.mockImplementation(() => {
+            return Promise.resolve(jsonResponse({
+                data: {
+                    user: {
+                        id: "123",
+                        contributionsCollection: {
+                            totalCommitContributions: 100,
+                            totalPullRequestContributions: 50,
+                            totalIssueContributions: 10,
+                            totalPullRequestReviewContributions: 0,
+                            contributionCalendar: { totalContributions: 160, weeks: [] },
+                        }
+                    }
+                }
+            }));
+        });
+
+        const data = await fetchYearInReviewData("testuser", 2023, "token");
+        expect(data.topRepository).toBeNull();
+    });
+
+    it("handles undefined repository data in mergeTopRepository", async () => {
+        mockFetch.mockImplementation(() => {
+            return Promise.resolve(jsonResponse({
+                data: {
+                    user: {
+                        id: "123",
+                        contributionsCollection: {
+                            totalCommitContributions: 100,
+                            totalPullRequestContributions: 50,
+                            totalIssueContributions: 10,
+                            totalPullRequestReviewContributions: 0,
+                            contributionCalendar: { totalContributions: 160, weeks: [] },
+                            commitContributionsByRepository: undefined,
+                            pullRequestContributionsByRepository: undefined,
+                            issueContributionsByRepository: undefined,
+                        }
+                    }
+                }
+            }));
+        });
+
+        const data = await fetchYearInReviewData("testuser", 2023, "token");
+        expect(data.topRepository).toBeNull();
+    });
+
+    it("handles full repository data in mergeTopRepository", async () => {
+        mockFetch.mockImplementation(() => {
+            return Promise.resolve(jsonResponse({
+                data: {
+                    user: {
+                        id: "123",
+                        contributionsCollection: {
+                            totalCommitContributions: 100,
+                            totalPullRequestContributions: 50,
+                            totalIssueContributions: 10,
+                            totalPullRequestReviewContributions: 0,
+                            contributionCalendar: { totalContributions: 160, weeks: [] },
+                            commitContributionsByRepository: [
+                                { repository: { owner: { login: "user1" }, name: "repo1" }, contributions: { totalCount: 50 } }
+                            ],
+                            pullRequestContributionsByRepository: [
+                                { repository: { owner: { login: "user1" }, name: "repo1" }, contributions: { totalCount: 20 } }
+                            ],
+                            issueContributionsByRepository: [
+                                { repository: { owner: { login: "user1" }, name: "repo1" }, contributions: { totalCount: 10 } }
+                            ],
+                        }
+                    }
+                }
+            }));
+        });
+
+        const data = await fetchYearInReviewData("testuser", 2023, "token");
+        expect(data.topRepository).toEqual({ name: "user1/repo1", contributions: 80 });
+    });
+
     it("handles partial repository data in mergeTopRepository", async () => {
         mockFetch.mockImplementation(() => {
             return Promise.resolve(jsonResponse({
diff --git a/src/lib/github.ts b/src/lib/github.ts
index d35c21b..ed7dd37 100644
--- a/src/lib/github.ts
+++ b/src/lib/github.ts
@@ -743,13 +743,24 @@ export const fetchActivity = cache(async function fetchActivity(
 
 /**
  * 効率的に Map から上位 K 件を抽出するヘルパー関数
- * 上位 K 件だけを返します。
+ * 配列の作成とソートを最小限に抑えることでパフォーマンスを向上させます
  */
 function getTopK(map: Map<string, number>, k: number = 10): { name: string; count: number }[] {
-  const limit = Math.max(0, k);
-  return Array.from(map, ([name, count]) => ({ name, count }))
-    .sort((a, b) => b.count - a.count)
-    .slice(0, limit);
+  const top: { name: string; count: number }[] = [];
+  for (const [name, count] of map.entries()) {
+    if (top.length < k) {
+      top.push({ name, count });
+      top.sort((a, b) => b.count - a.count);
+    } else if (count > top[k - 1].count) {
+      let i = k - 2;
+      while (i >= 0 && top[i].count < count) {
+        top[i + 1] = top[i];
+        i--;
+      }
+      top[i + 1] = { name, count };
+    }
+  }
+  return top;
 }
 
 /**
diff --git a/src/lib/githubYearInReview.ts b/src/lib/githubYearInReview.ts
index 23c4538..36fc68f 100644
--- a/src/lib/githubYearInReview.ts
+++ b/src/lib/githubYearInReview.ts
@@ -118,15 +118,19 @@ async function graphql<T>(query: string, token: string, variables: Record<string
 
 function mergeTopRepository(data: NonNullable<YearInReviewResponse["user"]>["contributionsCollection"]): { name: string; contributions: number } | null {
     const counter = new Map<string, number>();
+
     const buckets = [
-        ...(data.commitContributionsByRepository || []),
-        ...(data.pullRequestContributionsByRepository || []),
-        ...(data.issueContributionsByRepository || []),
+        data.commitContributionsByRepository,
+        data.pullRequestContributionsByRepository,
+        data.issueContributionsByRepository,
     ];
 
-    for (const item of buckets) {
-        const name = `${item.repository.owner.login}/${item.repository.name}`;
-        counter.set(name, (counter.get(name) ?? 0) + item.contributions.totalCount);
+    for (const bucket of buckets) {
+        if (!bucket) continue;
+        for (const item of bucket) {
+            const name = `${item.repository.owner.login}/${item.repository.name}`;
+            counter.set(name, (counter.get(name) ?? 0) + item.contributions.totalCount);
+        }
     }
 
     let top: { name: string; contributions: number } | null = null;
diff --git a/vitest.config.ts b/vitest.config.ts
index 9404856..eba2a2b 100644
--- a/vitest.config.ts
+++ b/vitest.config.ts
@@ -13,6 +13,7 @@ export default defineConfig({
       provider: "v8",
       reporter: ["text", "lcov"],
       include: [
+        "src/lib/githubYearInReview.ts",
         "src/lib/**/*.ts",
         "src/hooks/**/*.ts",
         "src/components/ThemeController.tsx",