diff --git a/compiler/mono/src/ir.rs b/compiler/mono/src/ir.rs
index b598eb48a9..379a26fa58 100644
--- a/compiler/mono/src/ir.rs
+++ b/compiler/mono/src/ir.rs
@@ -9,7 +9,7 @@ use crate::layout::{
 use bumpalo::collections::Vec;
 use bumpalo::Bump;
 use hashbrown::hash_map::Entry;
-use roc_collections::all::{default_hasher, BumpMap, BumpMapDefault, MutMap, MutSet};
+use roc_collections::all::{default_hasher, BumpMap, BumpMapDefault, MutMap};
 use roc_module::ident::{ForeignSymbol, Lowercase, TagName};
 use roc_module::low_level::LowLevel;
 use roc_module::symbol::{IdentIds, ModuleId, Symbol};
@@ -378,7 +378,8 @@ impl<'a> Proc<'a> {
 
 #[derive(Clone, Debug)]
 pub struct ExternalSpecializations<'a> {
-    pub specs: BumpMap<Symbol, MutSet<SolvedType>>,
+    /// Not a bumpalo vec because bumpalo is not thread safe
+    pub specs: BumpMap<Symbol, std::vec::Vec<SolvedType>>,
     _lifetime: std::marker::PhantomData<&'a u8>,
 }
 
@@ -394,11 +395,11 @@ impl<'a> ExternalSpecializations<'a> {
         use hashbrown::hash_map::Entry::{Occupied, Vacant};
 
         let existing = match self.specs.entry(symbol) {
-            Vacant(entry) => entry.insert(MutSet::default()),
+            Vacant(entry) => entry.insert(std::vec::Vec::new()),
             Occupied(entry) => entry.into_mut(),
         };
 
-        existing.insert(typ);
+        existing.push(typ);
     }
 
     pub fn extend(&mut self, other: Self) {
@@ -406,7 +407,7 @@ impl<'a> ExternalSpecializations<'a> {
 
         for (symbol, solved_types) in other.specs {
             let existing = match self.specs.entry(symbol) {
-                Vacant(entry) => entry.insert(MutSet::default()),
+                Vacant(entry) => entry.insert(std::vec::Vec::new()),
                 Occupied(entry) => entry.into_mut(),
             };
 
@@ -514,9 +515,12 @@ impl<'a> Procs<'a> {
                 // by the surrounding context, so we can add pending specializations
                 // for them immediately.
 
-                let tuple = (symbol, top_level);
-                let already_specialized = self.specialized.contains_key(&tuple);
-                let (symbol, layout) = tuple;
+                let already_specialized = self
+                    .specialized
+                    .keys()
+                    .any(|(s, t)| *s == symbol && *t == top_level);
+
+                let layout = top_level;
 
                 // if we've already specialized this one, no further work is needed.
                 if !already_specialized {
@@ -778,8 +782,6 @@ impl<'a, 'i> Env<'a, 'i> {
     pub fn unique_symbol(&mut self) -> Symbol {
         let ident_id = self.ident_ids.gen_unique();
 
-        self.home.register_debug_idents(self.ident_ids);
-
         Symbol::new(self.home, ident_id)
     }
 
@@ -1695,19 +1697,16 @@ pub fn specialize_all<'a>(
     env: &mut Env<'a, '_>,
     mut procs: Procs<'a>,
     externals_others_need: ExternalSpecializations<'a>,
-    pending_specializations: BumpMap<Symbol, MutMap<ProcLayout<'a>, PendingSpecialization<'a>>>,
+    specializations_for_host: BumpMap<Symbol, MutMap<ProcLayout<'a>, PendingSpecialization<'a>>>,
     layout_cache: &mut LayoutCache<'a>,
 ) -> Procs<'a> {
-    specialize_all_help(env, &mut procs, externals_others_need, layout_cache);
+    specialize_externals_others_need(env, &mut procs, externals_others_need, layout_cache);
 
     // When calling from_can, pending_specializations should be unavailable.
     // This must be a single pass, and we must not add any more entries to it!
-
-    // observation: specialize_all_help does add to pending_specializations, but does not reference
-    // any existing values in it.
     let opt_pending_specializations = std::mem::replace(&mut procs.pending_specializations, None);
 
-    let it = pending_specializations
+    let it = specializations_for_host
         .into_iter()
         .chain(opt_pending_specializations.into_iter().flatten());
 
@@ -1774,33 +1773,35 @@ pub fn specialize_all<'a>(
     procs
 }
 
-fn specialize_all_help<'a>(
+fn specialize_externals_others_need<'a>(
     env: &mut Env<'a, '_>,
     procs: &mut Procs<'a>,
     externals_others_need: ExternalSpecializations<'a>,
     layout_cache: &mut LayoutCache<'a>,
 ) {
     for (symbol, solved_types) in externals_others_need.specs.iter() {
-        // for some unclear reason, the MutSet does not deduplicate according to the hash
-        // instance. So we do it manually here
+        // de-duplicate by the Hash instance (set only deduplicates by Eq instance)
         use std::collections::hash_map::DefaultHasher;
         use std::hash::{Hash, Hasher};
 
+        let mut seen_hashes = Vec::with_capacity_in(solved_types.len(), env.arena);
+
         let hash_the_thing = |x: &SolvedType| {
             let mut hasher = DefaultHasher::new();
             x.hash(&mut hasher);
             hasher.finish()
         };
 
-        let mut as_vec = Vec::from_iter_in(
-            solved_types.iter().map(|x| (hash_the_thing(x), x)),
-            env.arena,
-        );
+        for solved_type in solved_types {
+            let hash = hash_the_thing(solved_type);
 
-        as_vec.sort_by_key(|(k, _)| *k);
-        as_vec.dedup_by_key(|(k, _)| *k);
+            if seen_hashes.iter().any(|h| *h == hash) {
+                // we've seen this one already
+                continue;
+            }
+
+            seen_hashes.push(hash);
 
-        for (_, solved_type) in as_vec {
             let name = *symbol;
 
             let partial_proc = match procs.partial_procs.get(&name) {
@@ -6535,7 +6536,8 @@ fn call_by_name_help<'a>(
     // If we've already specialized this one, no further work is needed.
     if procs
         .specialized
-        .contains_key(&(proc_name, top_level_layout))
+        .keys()
+        .any(|x| x == &(proc_name, top_level_layout))
     {
         debug_assert_eq!(
             argument_layouts.len(),