speeds O(n^2) execution of detectConflictingPaths (#46080)

## Bug

- [x] Related issues linked using `fixes #46079` #46079
- [ ] Integration tests added
- [ ] Errors have a helpful link attached, see [`contributing.md`](https://github.com/vercel/next.js/blob/canary/contributing.md)

## Feature

- [ ] Implements an existing feature request or RFC. Make sure the feature request has been accepted for implementation before opening a PR.
- [ ] Related issues linked using `fixes #number`
- [ ] [e2e](https://github.com/vercel/next.js/blob/canary/contributing/core/testing.md#writing-tests-for-nextjs) tests added
- [ ] Documentation added
- [ ] Telemetry added. In case of a feature if it's used or not.
- [ ] Errors have a helpful link attached, see [`contributing.md`](https://github.com/vercel/next.js/blob/canary/contributing.md)

## Documentation / Examples

- [ ] Make sure the linting passes by running `pnpm build && pnpm lint`
- [ ] The "examples guidelines" are followed from [our contributing doc](https://github.com/vercel/next.js/blob/canary/contributing/examples/adding-examples.md)

packages/next/src/build/utils.ts

@@ -1554,6 +1554,17 @@ export function detectConflictingPaths(
   >()
 
   const dynamicSsgPages = [...ssgPages].filter((page) => isDynamicRoute(page))
+  const additionalSsgPathsByPath: {
+    [page: string]: { [path: string]: string }
+  } = {}
+
+  additionalSsgPaths.forEach((paths, pathsPage) => {
+    additionalSsgPathsByPath[pathsPage] ||= {}
+    paths.forEach((curPath) => {
+      const currentPath = curPath.toLowerCase()
+      additionalSsgPathsByPath[pathsPage][currentPath] = curPath
+    })
+  })
 
   additionalSsgPaths.forEach((paths, pathsPage) => {
     paths.forEach((curPath) => {
@@ -1573,9 +1584,10 @@ export function detectConflictingPaths(
         conflictingPage = dynamicSsgPages.find((page) => {
           if (page === pathsPage) return false
 
-          conflictingPath = additionalSsgPaths
-            .get(page)
-            ?.find((compPath) => compPath.toLowerCase() === lowerPath)
+          conflictingPath =
+            additionalSsgPaths.get(page) == null
+              ? undefined
+              : additionalSsgPathsByPath[page][lowerPath]
           return conflictingPath
         })