diff --git a/src/bootstrap/format.rs b/src/bootstrap/format.rs
index 6d5753e8a6d4d..3637251256283 100644
--- a/src/bootstrap/format.rs
+++ b/src/bootstrap/format.rs
@@ -2,6 +2,7 @@
 
 use crate::builder::Builder;
 use crate::util::{output, program_out_of_date, t};
+use build_helper::ci::CiEnv;
 use build_helper::git::get_git_modified_files;
 use ignore::WalkBuilder;
 use std::collections::VecDeque;
@@ -156,7 +157,10 @@ pub fn format(build: &Builder<'_>, check: bool, paths: &[PathBuf]) {
                 // preventing the latter from being formatted.
                 ignore_fmt.add(&format!("!/{}", untracked_path)).expect(&untracked_path);
             }
-            if !check && paths.is_empty() {
+            // Only check modified files locally to speed up runtime.
+            // We still check all files in CI to avoid bugs in `get_modified_rs_files` letting regressions slip through;
+            // we also care about CI time less since this is still very fast compared to building the compiler.
+            if !CiEnv::is_ci() && paths.is_empty() {
                 match get_modified_rs_files(build) {
                     Ok(Some(files)) => {
                         for file in files {