diff --git a/Cargo.lock b/Cargo.lock
index ecdec9a6f..8e2cc8405 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -6108,6 +6108,7 @@ dependencies = [
  "mas-storage",
  "mas-storage-pg",
  "rand",
+ "rustc-hash 2.1.1",
  "serde",
  "sqlx",
  "thiserror 2.0.11",
diff --git a/crates/syn2mas/Cargo.toml b/crates/syn2mas/Cargo.toml
index 5b80b1510..578f705d2 100644
--- a/crates/syn2mas/Cargo.toml
+++ b/crates/syn2mas/Cargo.toml
@@ -23,6 +23,7 @@ chrono.workspace = true
 compact_str.workspace = true
 tracing.workspace = true
 futures-util = "0.3.31"
+rustc-hash = "2.1.1"
 
 rand.workspace = true
 uuid = "1.15.1"
diff --git a/crates/syn2mas/src/lib.rs b/crates/syn2mas/src/lib.rs
index a7a4b72ca..d0d1162fb 100644
--- a/crates/syn2mas/src/lib.rs
+++ b/crates/syn2mas/src/lib.rs
@@ -8,6 +8,9 @@ mod synapse_reader;
 
 mod migration;
 
+type RandomState = rustc_hash::FxBuildHasher;
+type HashMap<K, V> = rustc_hash::FxHashMap<K, V>;
+
 pub use self::{
     mas_writer::{MasWriter, checks::mas_pre_migration_checks, locking::LockedMasDatabase},
     migration::migrate,
diff --git a/crates/syn2mas/src/migration.rs b/crates/syn2mas/src/migration.rs
index f39c4a852..294baf0fc 100644
--- a/crates/syn2mas/src/migration.rs
+++ b/crates/syn2mas/src/migration.rs
@@ -11,7 +11,7 @@
 //! This module does not implement any of the safety checks that should be run
 //! *before* the migration.
 
-use std::{collections::HashMap, pin::pin, time::Instant};
+use std::{pin::pin, time::Instant};
 
 use chrono::{DateTime, Utc};
 use compact_str::CompactString;
@@ -25,7 +25,7 @@ use ulid::Ulid;
 use uuid::{NonNilUuid, Uuid};
 
 use crate::{
-    SynapseReader,
+    HashMap, RandomState, SynapseReader,
     mas_writer::{
         self, MasNewCompatAccessToken, MasNewCompatRefreshToken, MasNewCompatSession,
         MasNewEmailThreepid, MasNewUnsupportedThreepid, MasNewUpstreamOauthLink, MasNewUser,
@@ -114,7 +114,7 @@ struct MigrationState {
 
     /// A mapping of Synapse external ID providers to MAS upstream OAuth 2.0
     /// provider ID
-    provider_id_mapping: HashMap<String, Uuid>,
+    provider_id_mapping: std::collections::HashMap<String, Uuid>,
 }
 
 /// Performs a migration from Synapse's database to MAS' database.
@@ -136,7 +136,7 @@ pub async fn migrate(
     server_name: String,
     clock: &dyn Clock,
     rng: &mut impl RngCore,
-    provider_id_mapping: HashMap<String, Uuid>,
+    provider_id_mapping: std::collections::HashMap<String, Uuid>,
 ) -> Result<(), Error> {
     let counts = synapse.count_rows().await.into_synapse("counting users")?;
 
@@ -144,8 +144,11 @@ pub async fn migrate(
         server_name,
         // We oversize the hashmaps, as the estimates are innaccurate, and we would like to avoid
         // reallocations.
-        users: HashMap::with_capacity(counts.users * 9 / 8),
-        devices_to_compat_sessions: HashMap::with_capacity(counts.devices * 9 / 8),
+        users: HashMap::with_capacity_and_hasher(counts.users * 9 / 8, RandomState::default()),
+        devices_to_compat_sessions: HashMap::with_capacity_and_hasher(
+            counts.devices * 9 / 8,
+            RandomState::default(),
+        ),
         provider_id_mapping,
     };