diff --git a/core/src/subgraph/context/instance/mod.rs b/core/src/subgraph/context/instance/mod.rs
index 494ae227e97..9372791ccda 100644
--- a/core/src/subgraph/context/instance/mod.rs
+++ b/core/src/subgraph/context/instance/mod.rs
@@ -1,7 +1,7 @@
 mod hosts;
+mod module_cache;
 
 use anyhow::ensure;
-use graph::futures01::sync::mpsc::Sender;
 use graph::{
     blockchain::{Blockchain, TriggerData as _},
     data_source::{
@@ -11,7 +11,7 @@ use graph::{
     prelude::*,
 };
 use hosts::{OffchainHosts, OnchainHosts};
-use std::collections::HashMap;
+use module_cache::ModuleCache;
 
 pub(crate) struct SubgraphInstance<C: Blockchain, T: RuntimeHostBuilder<C>> {
     subgraph_id: DeploymentHash,
@@ -36,7 +36,7 @@ pub(crate) struct SubgraphInstance<C: Blockchain, T: RuntimeHostBuilder<C>> {
     offchain_hosts: OffchainHosts<C, T>,
 
     /// Maps the hash of a module to a channel to the thread in which the module is instantiated.
-    module_cache: HashMap<[u8; 32], Sender<T::Req>>,
+    module_cache: ModuleCache<T::Req>,
 
     /// This manages the sequence of causality regions for the subgraph.
     causality_region_seq: CausalityRegionSeq,
@@ -85,7 +85,7 @@ where
             onchain_hosts: OnchainHosts::new(),
             subgraph_hosts: OnchainHosts::new(),
             offchain_hosts: OffchainHosts::new(),
-            module_cache: HashMap::new(),
+            module_cache: ModuleCache::new(),
             templates,
             host_metrics,
             causality_region_seq,
@@ -104,7 +104,7 @@ where
         };
 
         let mapping_request_sender = {
-            let module_hash = alloy::primitives::keccak256(module_bytes.as_ref()).0;
+            let module_hash = self.module_cache.hash(&module_bytes);
             if let Some(sender) = self.module_cache.get(&module_hash) {
                 sender.clone()
             } else {
diff --git a/core/src/subgraph/context/instance/module_cache.rs b/core/src/subgraph/context/instance/module_cache.rs
new file mode 100644
index 00000000000..1440f557bee
--- /dev/null
+++ b/core/src/subgraph/context/instance/module_cache.rs
@@ -0,0 +1,189 @@
+use graph::futures01::sync::mpsc::Sender;
+use std::collections::HashMap;
+use std::sync::Arc;
+
+/// Maps the hash of a module to a channel to the thread in which the module
+/// is instantiated.
+///
+/// Hashing a module reads all of its bytes, and a subgraph can have millions
+/// of data sources that share a handful of modules. The hash is therefore
+/// memoized by the identity of the `Arc` that holds the bytes: every data
+/// source created from the same manifest mapping or template shares that
+/// mapping's `Arc`. The memo holds a clone of the `Arc`, so the allocation
+/// lives as long as the entry and an address found in the memo always
+/// designates that same `Arc`. Bytes in another `Arc` are hashed again: equal
+/// contents give the same hash, hence the same module.
+pub(super) struct ModuleCache<R> {
+    senders: HashMap<[u8; 32], Sender<R>>,
+    hashes: HashMap<usize, (Arc<Vec<u8>>, [u8; 32])>,
+}
+
+impl<R> ModuleCache<R> {
+    pub fn new() -> Self {
+        ModuleCache {
+            senders: HashMap::new(),
+            hashes: HashMap::new(),
+        }
+    }
+
+    /// The keccak256 hash of `module_bytes`, computed once per `Arc`.
+    pub fn hash(&mut self, module_bytes: &Arc<Vec<u8>>) -> [u8; 32] {
+        let key = Arc::as_ptr(module_bytes) as usize;
+        if let Some((_, hash)) = self.hashes.get(&key) {
+            return *hash;
+        }
+        let hash = alloy::primitives::keccak256(module_bytes.as_slice()).0;
+        self.hashes.insert(key, (module_bytes.clone(), hash));
+        hash
+    }
+
+    pub fn get(&self, module_hash: &[u8; 32]) -> Option<&Sender<R>> {
+        self.senders.get(module_hash)
+    }
+
+    pub fn insert(&mut self, module_hash: [u8; 32], sender: Sender<R>) {
+        self.senders.insert(module_hash, sender);
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use graph::futures01::sync::mpsc::{Receiver, channel};
+
+    /// A cache used like `new_host` uses it, whose spawns are recorded: the
+    /// receivers, by spawn order.
+    struct Spawns {
+        cache: ModuleCache<u32>,
+        receivers: Vec<Receiver<u32>>,
+    }
+
+    impl Spawns {
+        fn new() -> Self {
+            Spawns {
+                cache: ModuleCache::new(),
+                receivers: Vec::new(),
+            }
+        }
+
+        /// Memoize `mark` as the hash of `bytes`.
+        fn seed(&mut self, bytes: &Arc<Vec<u8>>, mark: [u8; 32]) {
+            let key = Arc::as_ptr(bytes) as usize;
+            self.cache.hashes.insert(key, (bytes.clone(), mark));
+        }
+
+        fn sender(&mut self, bytes: &Arc<Vec<u8>>) -> Sender<u32> {
+            let module_hash = self.cache.hash(bytes);
+            if let Some(sender) = self.cache.get(&module_hash) {
+                return sender.clone();
+            }
+            let (tx, rx) = channel(1);
+            self.receivers.push(rx);
+            self.cache.insert(module_hash, tx.clone());
+            tx
+        }
+    }
+
+    /// Whether `sender` sends to the receiver of spawn `idx`: drop that
+    /// receiver and see if the sender is disconnected.
+    fn sends_to(mut spawns: Spawns, sender: &mut Sender<u32>, idx: usize) -> bool {
+        drop(spawns.receivers.remove(idx));
+        sender.try_send(0).is_err_and(|e| e.is_disconnected())
+    }
+
+    fn module(n: u8) -> Vec<u8> {
+        (0..66_000u32).map(|i| (i as u8) ^ n).collect()
+    }
+
+    #[test]
+    fn two_templates_two_hashes() {
+        let a = Arc::new(module(1));
+        let b = Arc::new(module(2));
+        let mut s = Spawns::new();
+        let ha = s.cache.hash(&a);
+        let hb = s.cache.hash(&b);
+        assert_ne!(ha, hb);
+        assert_eq!(ha, alloy::primitives::keccak256(a.as_slice()).0);
+        assert_eq!(hb, alloy::primitives::keccak256(b.as_slice()).0);
+
+        s.sender(&a);
+        let mut sb = s.sender(&b);
+        assert_eq!(s.receivers.len(), 2, "one module per template");
+        assert!(sends_to(s, &mut sb, 1));
+    }
+
+    #[test]
+    fn sources_of_one_template_share_hash_and_sender() {
+        let template = Arc::new(module(1));
+        let other = Arc::new(module(2));
+        let mut s = Spawns::new();
+        s.sender(&other);
+        s.sender(&template);
+        let mut second = s.sender(&template.clone());
+        assert_eq!(s.cache.hashes.len(), 2);
+        assert_eq!(s.receivers.len(), 2);
+        assert!(sends_to(s, &mut second, 1), "the template's sender");
+    }
+
+    #[test]
+    fn many_sources_hash_once_per_template() {
+        const N: usize = 100_000;
+        // A recomputed hash overwrites its entry, which `hashes.len()` can't
+        // show. Marks are not real hashes on purpose: only a hit returns them.
+        let (mark_a, mark_b) = ([0xaa; 32], [0xbb; 32]);
+        let a = Arc::new(module(1));
+        let b = Arc::new(module(2));
+        let mut s = Spawns::new();
+        s.seed(&a, mark_a);
+        s.seed(&b, mark_b);
+        for i in 0..N {
+            let (bytes, mark) = if i % 2 == 0 {
+                (a.clone(), mark_a)
+            } else {
+                (b.clone(), mark_b)
+            };
+            assert_eq!(s.cache.hash(&bytes), mark);
+            s.sender(&bytes);
+        }
+        assert_eq!(s.cache.hashes.len(), 2);
+        assert_eq!(s.receivers.len(), 2);
+
+        let mut s = Spawns::new();
+        for _ in 0..N {
+            s.sender(&a.clone());
+        }
+        assert_eq!(s.cache.hashes.len(), 1);
+        assert_eq!(s.receivers.len(), 1);
+    }
+
+    #[test]
+    fn equal_unshared_bytes_give_same_hash_and_module() {
+        let shared = Arc::new(module(1));
+        let copy = Arc::new(module(1));
+        assert!(!Arc::ptr_eq(&shared, &copy));
+        let mut s = Spawns::new();
+        s.sender(&Arc::new(module(2)));
+        s.sender(&shared);
+        let hash = s.cache.hash(&shared);
+        assert_eq!(s.cache.hash(&copy), hash);
+        assert_eq!(s.cache.hashes.len(), 3, "the copy is hashed");
+        let mut from_copy = s.sender(&copy);
+        assert_eq!(s.receivers.len(), 2, "no new module for the copy");
+        assert!(sends_to(s, &mut from_copy, 1), "the shared bytes' sender");
+    }
+
+    #[test]
+    fn dropped_bytes_are_not_confused_with_new_ones() {
+        // Allocate and drop many `Arc`s: the memo holds each one, so no
+        // later allocation can take the address of a memoized one.
+        let mut s = Spawns::new();
+        for n in 0..64u8 {
+            let bytes = Arc::new(vec![n; 64]);
+            assert_eq!(
+                s.cache.hash(&bytes),
+                alloy::primitives::keccak256(bytes.as_slice()).0
+            );
+        }
+        assert_eq!(s.cache.hashes.len(), 64);
+    }
+}