diff --git a/compiler/rustc_ast_lowering/src/lib.rs b/compiler/rustc_ast_lowering/src/lib.rs
index 1e14b4d6723f4..8b1c63cd21d8c 100644
--- a/compiler/rustc_ast_lowering/src/lib.rs
+++ b/compiler/rustc_ast_lowering/src/lib.rs
@@ -49,6 +49,7 @@ use rustc_attr_parsing::{AttributeParser, OmitDoc};
 use rustc_data_structures::fingerprint::Fingerprint;
 use rustc_data_structures::sorted_map::SortedMap;
 use rustc_data_structures::stable_hasher::{HashStable, StableHasher};
+use rustc_data_structures::sync::spawn;
 use rustc_data_structures::tagged_ptr::TaggedRef;
 use rustc_errors::{DiagArgFromDisplay, DiagCtxtHandle, StashKey};
 use rustc_hir::def::{DefKind, LifetimeRes, Namespace, PartialRes, PerNS, Res};
@@ -454,9 +455,14 @@ pub fn lower_to_hir(tcx: TyCtxt<'_>, (): ()) -> hir::Crate<'_> {
         .lower_node(def_id);
     }
 
-    // Drop AST to free memory
     drop(ast_index);
-    sess.time("drop_ast", || drop(krate));
+
+    // Drop AST to free memory. It can be expensive so try to drop it on a separate thread.
+    let prof = sess.prof.clone();
+    spawn(move || {
+        let _timer = prof.verbose_generic_activity("drop_ast");
+        drop(krate);
+    });
 
     // Don't hash unless necessary, because it's expensive.
     let opt_hir_hash =
diff --git a/compiler/rustc_data_structures/src/sync.rs b/compiler/rustc_data_structures/src/sync.rs
index 616a18a72ab7e..80d49effbf8a9 100644
--- a/compiler/rustc_data_structures/src/sync.rs
+++ b/compiler/rustc_data_structures/src/sync.rs
@@ -43,7 +43,7 @@ pub use self::freeze::{FreezeLock, FreezeReadGuard, FreezeWriteGuard};
 pub use self::lock::{Lock, LockGuard, Mode};
 pub use self::mode::{is_dyn_thread_safe, set_dyn_thread_safe_mode};
 pub use self::parallel::{
-    join, par_for_each_in, par_map, parallel_guard, scope, try_par_for_each_in,
+    join, par_for_each_in, par_map, parallel_guard, scope, spawn, try_par_for_each_in,
 };
 pub use self::vec::{AppendOnlyIndexVec, AppendOnlyVec};
 pub use self::worker_local::{Registry, WorkerLocal};
diff --git a/compiler/rustc_data_structures/src/sync/parallel.rs b/compiler/rustc_data_structures/src/sync/parallel.rs
index ba3c85ef5b155..64db39cc4c6e4 100644
--- a/compiler/rustc_data_structures/src/sync/parallel.rs
+++ b/compiler/rustc_data_structures/src/sync/parallel.rs
@@ -93,6 +93,17 @@ macro_rules! parallel {
         };
     }
 
+pub fn spawn(func: impl FnOnce() + DynSend + 'static) {
+    if mode::is_dyn_thread_safe() {
+        let func = FromDyn::from(func);
+        rayon_core::spawn(|| {
+            (func.into_inner())();
+        });
+    } else {
+        func()
+    }
+}
+
 // This function only works when `mode::is_dyn_thread_safe()`.
 pub fn scope<'scope, OP, R>(op: OP) -> R
 where
diff --git a/compiler/rustc_interface/src/passes.rs b/compiler/rustc_interface/src/passes.rs
index 66d2a79b93a4a..aad63f4edcf3e 100644
--- a/compiler/rustc_interface/src/passes.rs
+++ b/compiler/rustc_interface/src/passes.rs
@@ -900,6 +900,12 @@ fn run_required_analyses(tcx: TyCtxt<'_>) {
     // is not defined. So we need to cfg it out.
     #[cfg(all(not(doc), debug_assertions))]
     rustc_passes::hir_id_validator::check_crate(tcx);
+
+    // Prefetch this to prevent multiple threads from blocking on it later.
+    // This is needed since the `hir_id_validator::check_crate` call above is not guaranteed
+    // to use `hir_crate`.
+    tcx.ensure_done().hir_crate(());
+
     let sess = tcx.sess;
     sess.time("misc_checking_1", || {
         parallel!(