diff --git a/Cargo.lock b/Cargo.lock
index aca639d79a..a373df043a 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -2076,6 +2076,16 @@ version = "2.4.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "308cc39be01b73d0d18f82a0e7b2a3df85245f84af96fdddc5d202d27e47b86a"
 
+[[package]]
+name = "memmap"
+version = "0.7.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "6585fd95e7bb50d6cc31e20d4cf9afb4e2ba16c5846fc76793f11218da9c475b"
+dependencies = [
+ "libc",
+ "winapi",
+]
+
 [[package]]
 name = "memmap2"
 version = "0.1.0"
@@ -3677,6 +3687,7 @@ dependencies = [
  "crossbeam",
  "indoc",
  "maplit",
+ "memmap",
  "morphic_lib",
  "num_cpus",
  "parking_lot",
diff --git a/compiler/load/Cargo.toml b/compiler/load/Cargo.toml
index 3945650771..afb62d8b77 100644
--- a/compiler/load/Cargo.toml
+++ b/compiler/load/Cargo.toml
@@ -27,6 +27,7 @@ bumpalo = { version = "3.8.0", features = ["collections"] }
 parking_lot = "0.11.2"
 crossbeam = "0.8.1"
 num_cpus = "1.13.0"
+memmap = "0.7.0"
 
 [dev-dependencies]
 tempfile = "3.2.0"
diff --git a/compiler/load/src/file.rs b/compiler/load/src/file.rs
index 76586c333c..5eeb735622 100644
--- a/compiler/load/src/file.rs
+++ b/compiler/load/src/file.rs
@@ -4475,29 +4475,67 @@ fn run_solve<'a>(
         solve_aliases.insert(*name, alias.clone());
     }
 
-    let (solved_subs, solved_env, problems) = roc_solve::module::run_solve(
-        &constraints,
-        actual_constraint,
-        rigid_variables,
-        subs,
-        solve_aliases,
-    );
+    let (solved_subs, exposed_vars_by_symbol, problems) = if module_id.is_builtin() {
+        use memmap::MmapOptions;
+        use std::fs::File;
 
-    let solved_subs = if true {
-        solved_subs
+        // let file = File::open(&format!("cached_subs/{:?}.dat", module_id)).unwrap();
+        // let mmap = unsafe { MmapOptions::new().map(&file).unwrap() };
+
+        let mmap = std::fs::read(&format!("cached_subs/{:?}.dat", module_id)).unwrap();
+        println!("started {:?}", module_id);
+        let (subs, vars_by_symbol) = Subs::deserialize(&mmap);
+        println!("deserialized");
+        let solved_subs = Solved(subs);
+
+        let exposed_vars_by_symbol: Vec<_> = vars_by_symbol
+            .into_iter()
+            .filter(|(k, _)| exposed_symbols.contains(k))
+            .collect();
+
+        (solved_subs, exposed_vars_by_symbol, vec![])
     } else {
-        let mut serialized = Vec::new();
-        solved_subs.inner().serialize(&mut serialized).unwrap();
-        let subs = Subs::deserialize(&serialized);
+        let (solved_subs, solved_env, problems) = roc_solve::module::run_solve(
+            &constraints,
+            actual_constraint,
+            rigid_variables,
+            subs,
+            solve_aliases,
+        );
 
-        Solved(subs)
+        if module_id.is_builtin() {
+            let mut f = std::fs::File::create(&format!("cached_subs/{:?}.dat", module_id)).unwrap();
+            let vars_by_symbol: Vec<(Symbol, Variable)> = solved_env.vars_by_symbol().collect();
+            solved_subs
+                .inner()
+                .serialize(&vars_by_symbol, &mut f)
+                .unwrap();
+        }
+
+        let solved_subs = if true {
+            solved_subs
+        } else {
+            let vars_by_symbol: Vec<(Symbol, Variable)> = solved_env.vars_by_symbol().collect();
+            let mut serialized = Vec::new();
+            solved_subs
+                .inner()
+                .serialize(&vars_by_symbol, &mut serialized)
+                .unwrap();
+            let (subs, vbs) = Subs::deserialize(&serialized);
+
+            dbg!(vbs);
+
+            Solved(subs)
+        };
+
+        let exposed_vars_by_symbol: Vec<_> = solved_env
+            .vars_by_symbol()
+            .filter(|(k, _)| exposed_symbols.contains(k))
+            .collect();
+
+        (solved_subs, exposed_vars_by_symbol, problems)
     };
 
-    let exposed_vars_by_symbol: Vec<_> = solved_env
-        .vars_by_symbol()
-        .filter(|(k, _)| exposed_symbols.contains(k))
-        .collect();
-
     let mut solved_subs = solved_subs;
     let (storage_subs, stored_vars_by_symbol) =
         roc_solve::module::exposed_types_storage_subs(&mut solved_subs, &exposed_vars_by_symbol);
diff --git a/compiler/types/src/subs.rs b/compiler/types/src/subs.rs
index ff77bc0683..e9e4aac075 100644
--- a/compiler/types/src/subs.rs
+++ b/compiler/types/src/subs.rs
@@ -69,10 +69,11 @@ struct SubsHeader {
     field_names: usize,
     record_fields: usize,
     variable_slices: usize,
+    vars_by_symbol: usize,
 }
 
 impl SubsHeader {
-    fn from_subs(subs: &Subs) -> Self {
+    fn from_subs(subs: &Subs, vars_by_symbol: usize) -> Self {
         // TODO what do we do with problems? they should
         // be reported and then removed from Subs I think
         debug_assert!(subs.problems.is_empty());
@@ -84,6 +85,7 @@ impl SubsHeader {
             field_names: subs.field_names.len(),
             record_fields: subs.record_fields.len(),
             variable_slices: subs.variable_slices.len(),
+            vars_by_symbol,
         }
     }
 
@@ -108,10 +110,14 @@ fn round_to_multiple_of(value: usize, base: usize) -> usize {
 }
 
 impl Subs {
-    pub fn serialize(&self, writer: &mut impl std::io::Write) -> std::io::Result<usize> {
+    pub fn serialize(
+        &self,
+        vars_by_symbol: &[(Symbol, Variable)],
+        writer: &mut impl std::io::Write,
+    ) -> std::io::Result<usize> {
         let mut written = 0;
 
-        let header = SubsHeader::from_subs(self).to_array();
+        let header = SubsHeader::from_subs(self, vars_by_symbol.len()).to_array();
         written += header.len();
         writer.write_all(&header)?;
 
@@ -122,6 +128,7 @@ impl Subs {
         written = Self::serialize_field_names(&self.field_names, writer, written)?;
         written = Self::serialize_slice(&self.record_fields, writer, written)?;
         written = Self::serialize_slice(&self.variable_slices, writer, written)?;
+        written = Self::serialize_slice(vars_by_symbol, writer, written)?;
 
         Ok(written)
     }
@@ -195,7 +202,7 @@ impl Subs {
         Ok(written + padding_bytes + bytes_slice.len())
     }
 
-    pub fn deserialize(bytes: &[u8]) -> Self {
+    pub fn deserialize(bytes: &[u8]) -> (Self, Vec<(Symbol, Variable)>) {
         use std::convert::TryInto;
 
         let mut offset = 0;
@@ -205,14 +212,21 @@ impl Subs {
 
         let (utable, offset) = Self::deserialize_unification_table(bytes, header.utable, offset);
 
+        println!("utable");
+
         let (variables, offset) = Self::deserialize_slice(bytes, header.variables, offset);
         let (tag_names, offset) = Self::deserialize_slice(bytes, header.tag_names, offset);
         let (field_names, offset) =
             Self::deserialize_field_names(bytes, header.field_names, offset);
+        println!("field names");
         let (record_fields, offset) = Self::deserialize_slice(bytes, header.record_fields, offset);
-        let (variable_slices, _) = Self::deserialize_slice(bytes, header.variable_slices, offset);
+        let (variable_slices, offset) =
+            Self::deserialize_slice(bytes, header.variable_slices, offset);
+        let (vars_by_symbol, _) = Self::deserialize_slice(bytes, header.vars_by_symbol, offset);
+        println!("all");
+        let x = vars_by_symbol.to_vec();
 
-        Self {
+        let subs = Self {
             utable,
             variables: variables.to_vec(),
             tag_names: tag_names.to_vec(),
@@ -221,7 +235,10 @@ impl Subs {
             variable_slices: variable_slices.to_vec(),
             tag_name_cache: Default::default(),
             problems: Default::default(),
-        }
+        };
+        println!("to_vec");
+
+        (subs, x)
     }
 
     fn deserialize_unification_table(