diff --git a/src/librustc_mir/monomorphize/collector.rs b/src/librustc_mir/monomorphize/collector.rs
index 5ccdb4be62461..f5f00c9343561 100644
--- a/src/librustc_mir/monomorphize/collector.rs
+++ b/src/librustc_mir/monomorphize/collector.rs
@@ -194,7 +194,7 @@ use rustc_hir as hir;
 use rustc_hir::def_id::{DefId, DefIdMap, LOCAL_CRATE};
 use rustc_hir::itemlikevisit::ItemLikeVisitor;
 use rustc_index::bit_set::GrowableBitSet;
-
+use smallvec::SmallVec;
 use std::iter;
 
 #[derive(PartialEq)]
@@ -227,12 +227,7 @@ impl<'tcx> InliningMap<'tcx> {
         }
     }
 
-    fn record_accesses<I>(&mut self, source: MonoItem<'tcx>, new_targets: I)
-    where
-        I: Iterator<Item = (MonoItem<'tcx>, bool)> + ExactSizeIterator,
-    {
-        assert!(!self.index.contains_key(&source));
-
+    fn record_accesses(&mut self, source: MonoItem<'tcx>, new_targets: &[(MonoItem<'tcx>, bool)]) {
         let start_index = self.targets.len();
         let new_items_count = new_targets.len();
         let new_items_count_total = new_items_count + self.targets.len();
@@ -240,15 +235,15 @@ impl<'tcx> InliningMap<'tcx> {
         self.targets.reserve(new_items_count);
         self.inlines.ensure(new_items_count_total);
 
-        for (i, (target, inline)) in new_targets.enumerate() {
-            self.targets.push(target);
-            if inline {
+        for (i, (target, inline)) in new_targets.iter().enumerate() {
+            self.targets.push(*target);
+            if *inline {
                 self.inlines.insert(i + start_index);
             }
         }
 
         let end_index = self.targets.len();
-        self.index.insert(source, (start_index, end_index));
+        assert!(self.index.insert(source, (start_index, end_index)).is_none());
     }
 
     // Internally iterate over all items referenced by `source` which will be
@@ -403,10 +398,15 @@ fn record_accesses<'tcx>(
         mono_item.instantiation_mode(tcx) == InstantiationMode::LocalCopy
     };
 
-    let accesses =
-        callees.into_iter().map(|mono_item| (*mono_item, is_inlining_candidate(mono_item)));
+    // We collect this into a `SmallVec` to avoid calling `is_inlining_candidate` in the lock.
+    // FIXME: Call `is_inlining_candidate` when pushing to `neighbors` in `collect_items_rec`
+    // instead to avoid creating this `SmallVec`.
+    let accesses: SmallVec<[_; 128]> = callees
+        .into_iter()
+        .map(|mono_item| (*mono_item, is_inlining_candidate(mono_item)))
+        .collect();
 
-    inlining_map.lock_mut().record_accesses(caller, accesses);
+    inlining_map.lock_mut().record_accesses(caller, &accesses);
 }
 
 fn check_recursion_limit<'tcx>(
diff --git a/src/librustc_mir/monomorphize/partitioning.rs b/src/librustc_mir/monomorphize/partitioning.rs
index 8fa41cab19076..0def51a6a33e5 100644
--- a/src/librustc_mir/monomorphize/partitioning.rs
+++ b/src/librustc_mir/monomorphize/partitioning.rs
@@ -104,6 +104,7 @@ use rustc::ty::print::characteristic_def_id_of_type;
 use rustc::ty::query::Providers;
 use rustc::ty::{self, DefIdTree, InstanceDef, TyCtxt};
 use rustc_data_structures::fx::{FxHashMap, FxHashSet};
+use rustc_data_structures::sync;
 use rustc_hir::def::DefKind;
 use rustc_hir::def_id::{CrateNum, DefId, DefIdSet, CRATE_DEF_INDEX, LOCAL_CRATE};
 use rustc_span::symbol::Symbol;
@@ -796,6 +797,8 @@ where
     I: Iterator<Item = &'a MonoItem<'tcx>>,
     'tcx: 'a,
 {
+    let _prof_timer = tcx.prof.generic_activity("assert_symbols_are_distinct");
+
     let mut symbols: Vec<_> =
         mono_items.map(|mono_item| (mono_item, mono_item.symbol_name(tcx))).collect();
 
@@ -869,18 +872,23 @@ fn collect_and_partition_mono_items(
 
     tcx.sess.abort_if_errors();
 
-    assert_symbols_are_distinct(tcx, items.iter());
-
-    let strategy = if tcx.sess.opts.incremental.is_some() {
-        PartitioningStrategy::PerModule
-    } else {
-        PartitioningStrategy::FixedUnitCount(tcx.sess.codegen_units())
-    };
-
-    let codegen_units = partition(tcx, items.iter().cloned(), strategy, &inlining_map)
-        .into_iter()
-        .map(Arc::new)
-        .collect::<Vec<_>>();
+    let (codegen_units, _) = tcx.sess.time("partition_and_assert_distinct_symbols", || {
+        sync::join(
+            || {
+                let strategy = if tcx.sess.opts.incremental.is_some() {
+                    PartitioningStrategy::PerModule
+                } else {
+                    PartitioningStrategy::FixedUnitCount(tcx.sess.codegen_units())
+                };
+
+                partition(tcx, items.iter().cloned(), strategy, &inlining_map)
+                    .into_iter()
+                    .map(Arc::new)
+                    .collect::<Vec<_>>()
+            },
+            || assert_symbols_are_distinct(tcx, items.iter()),
+        )
+    });
 
     let mono_items: DefIdSet = items
         .iter()