diff --git a/Cargo.lock b/Cargo.lock
index 919e9a240..ddf5052cf 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -752,6 +752,19 @@ dependencies = [
  "num_cpus",
 ]
 
+[[package]]
+name = "dashmap"
+version = "5.5.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "978747c1d849a7d2ee5e8adc0159961c48fb7e5db2f06af6723b80123bb53856"
+dependencies = [
+ "cfg-if 1.0.0",
+ "hashbrown 0.14.3",
+ "lock_api",
+ "once_cell",
+ "parking_lot_core 0.9.9",
+]
+
 [[package]]
 name = "data-encoding"
 version = "2.5.0"
@@ -2648,6 +2661,7 @@ dependencies = [
  "cache-key",
  "chrono",
  "clap",
+ "dashmap 5.5.3",
  "derivative",
  "distribution-filename",
  "distribution-types",
@@ -4110,7 +4124,7 @@ version = "1.1.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "28491611b6b9a0b9f027be139a4be792b13a20780100dd8b054d44dbf596d52b"
 dependencies = [
- "dashmap",
+ "dashmap 3.11.10",
  "smallvec",
 ]
 
diff --git a/Cargo.toml b/Cargo.toml
index 25fa6038a..2492f1300 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -26,6 +26,7 @@ chrono = { version = "0.4.31" }
 clap = { version = "4.4.13" }
 configparser = { version = "3.0.4" }
 csv = { version = "1.3.0" }
+dashmap = { version = "5.5.3" }
 data-encoding = { version = "2.5.0" }
 derivative = { version = "2.2.0" }
 directories = { version = "5.0.1" }
diff --git a/crates/puffin-resolver/Cargo.toml b/crates/puffin-resolver/Cargo.toml
index 7f0705503..97e6726de 100644
--- a/crates/puffin-resolver/Cargo.toml
+++ b/crates/puffin-resolver/Cargo.toml
@@ -37,6 +37,7 @@ anyhow = { workspace = true }
 bitflags = { workspace = true }
 chrono = { workspace = true }
 clap = { workspace = true, features = ["derive"], optional = true }
+dashmap = { workspace = true }
 derivative = { workspace = true }
 fs-err = { workspace = true, features = ["tokio"] }
 futures = { workspace = true }
diff --git a/crates/puffin-resolver/src/resolver/index.rs b/crates/puffin-resolver/src/resolver/index.rs
index 84c49bd74..6976f6c99 100644
--- a/crates/puffin-resolver/src/resolver/index.rs
+++ b/crates/puffin-resolver/src/resolver/index.rs
@@ -1,7 +1,6 @@
 use url::Url;
 
 use distribution_types::PackageId;
-use pep440_rs::VersionSpecifiers;
 use puffin_normalize::PackageName;
 use puffin_traits::OnceMap;
 use pypi_types::Metadata21;
@@ -18,9 +17,6 @@ pub(crate) struct Index {
     /// A map from package ID to metadata for that distribution.
     pub(crate) distributions: OnceMap<PackageId, Metadata21>,
 
-    /// A map from package ID to required Python version.
-    pub(crate) incompatibilities: OnceMap<PackageId, VersionSpecifiers>,
-
     /// A map from source URL to precise URL.
     pub(crate) redirects: OnceMap<Url, Url>,
 }
@@ -32,7 +28,6 @@ impl Index {
     pub(crate) fn cancel_all(&self) {
         self.packages.cancel_all();
         self.distributions.cancel_all();
-        self.incompatibilities.cancel_all();
         self.redirects.cancel_all();
     }
 }
diff --git a/crates/puffin-resolver/src/resolver/mod.rs b/crates/puffin-resolver/src/resolver/mod.rs
index 2df666073..3b25e702f 100644
--- a/crates/puffin-resolver/src/resolver/mod.rs
+++ b/crates/puffin-resolver/src/resolver/mod.rs
@@ -3,6 +3,7 @@
 use std::sync::Arc;
 
 use anyhow::Result;
+use dashmap::DashMap;
 use futures::channel::mpsc::UnboundedReceiver;
 use futures::{pin_mut, FutureExt, StreamExt};
 use itertools::Itertools;
@@ -17,9 +18,10 @@ use url::Url;
 
 use distribution_filename::WheelFilename;
 use distribution_types::{
-    BuiltDist, Dist, DistributionMetadata, LocalEditable, Name, RemoteSource, SourceDist,
-    VersionOrUrl,
+    BuiltDist, Dist, DistributionMetadata, LocalEditable, Name, PackageId, RemoteSource,
+    SourceDist, VersionOrUrl,
 };
+use pep440_rs::VersionSpecifiers;
 use pep508_rs::{MarkerEnvironment, Requirement};
 use platform_tags::Tags;
 use puffin_client::RegistryClient;
@@ -64,6 +66,7 @@ pub struct Resolver<'a, Provider: ResolverProvider> {
     python_requirement: PythonRequirement<'a>,
     selector: CandidateSelector,
     index: Arc<Index>,
+    incompatibilities: DashMap<PackageId, VersionSpecifiers>,
     editables: FxHashMap<PackageName, (LocalEditable, Metadata21)>,
     reporter: Option<Arc<dyn Reporter>>,
     provider: Provider,
@@ -155,6 +158,7 @@ impl<'a, Provider: ResolverProvider> Resolver<'a, Provider> {
 
         Self {
             index: Arc::new(index),
+            incompatibilities: DashMap::default(),
             selector,
             allowed_urls,
             project: manifest.project,
@@ -492,9 +496,8 @@ impl<'a, Provider: ResolverProvider> Resolver<'a, Provider> {
 
                 // If the version is incompatible, short-circuit.
                 if let Some(requires_python) = candidate.validate(&self.python_requirement) {
-                    self.index
-                        .incompatibilities
-                        .done(candidate.package_id(), requires_python.clone());
+                    self.incompatibilities
+                        .insert(candidate.package_id(), requires_python.clone());
                     return Ok(Some(candidate.version().clone()));
                 }
 
@@ -606,7 +609,7 @@ impl<'a, Provider: ResolverProvider> Resolver<'a, Provider> {
 
                 // If the package is known to be incompatible, return the Python version as an
                 // incompatibility, and skip fetching the metadata.
-                if let Some(entry) = self.index.incompatibilities.get(&package_id) {
+                if let Some(entry) = self.incompatibilities.get(&package_id) {
                     let requires_python = entry.value();
                     let version = requires_python
                         .iter()
@@ -765,9 +768,8 @@ impl<'a, Provider: ResolverProvider> Resolver<'a, Provider> {
 
                 // If the version is incompatible, short-circuit.
                 if let Some(requires_python) = candidate.validate(&self.python_requirement) {
-                    self.index
-                        .incompatibilities
-                        .done(candidate.package_id(), requires_python.clone());
+                    self.incompatibilities
+                        .insert(candidate.package_id(), requires_python.clone());
                     return Ok(None);
                 }