diff --git a/Cargo.lock b/Cargo.lock
index 70c88018..9077b511 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -1047,6 +1047,32 @@ dependencies = [
  "cfg-if",
 ]
 
+[[package]]
+name = "criterion"
+version = "0.5.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "f2b12d017a929603d80db1831cd3a24082f8137ce19c69e6447f54f5fc8d692f"
+dependencies = [
+ "anes",
+ "cast",
+ "ciborium",
+ "clap",
+ "criterion-plot 0.5.0",
+ "is-terminal",
+ "itertools 0.10.5",
+ "num-traits",
+ "once_cell",
+ "oorandom",
+ "plotters",
+ "rayon",
+ "regex",
+ "serde",
+ "serde_derive",
+ "serde_json",
+ "tinytemplate",
+ "walkdir",
+]
+
 [[package]]
 name = "criterion"
 version = "0.8.2"
@@ -1058,7 +1084,7 @@ dependencies = [
  "cast",
  "ciborium",
  "clap",
- "criterion-plot",
+ "criterion-plot 0.8.2",
  "itertools 0.13.0",
  "num-traits",
  "oorandom",
@@ -1072,6 +1098,16 @@ dependencies = [
  "walkdir",
 ]
 
+[[package]]
+name = "criterion-plot"
+version = "0.5.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "6b50826342786a51a89e2da3a28f1c32b06e387201bc2d19791f622c673706b1"
+dependencies = [
+ "cast",
+ "itertools 0.10.5",
+]
+
 [[package]]
 name = "criterion-plot"
 version = "0.8.2"
@@ -2033,12 +2069,32 @@ dependencies = [
  "syn 2.0.119",
 ]
 
+[[package]]
+name = "is-terminal"
+version = "0.4.17"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "3640c1c38b8e4e43584d8df18be5fc6b0aa314ce6ebf51b53313d4306cca8e46"
+dependencies = [
+ "hermit-abi",
+ "libc",
+ "windows-sys 0.61.2",
+]
+
 [[package]]
 name = "is_terminal_polyfill"
 version = "1.70.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "a6cb138bb79a146c1bd460005623e142ef0181e3d0219cb493e02f7d08a35695"
 
+[[package]]
+name = "itertools"
+version = "0.10.5"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "b0fd2260e829bddf4cb6ea802289de2f86d6a7a690192fbe91b3f46e0f2c8473"
+dependencies = [
+ "either",
+]
+
 [[package]]
 name = "itertools"
 version = "0.13.0"
@@ -3894,6 +3950,7 @@ dependencies = [
 name = "techscript_ir"
 version = "2.0.0"
 dependencies = [
+ "criterion 0.5.1",
  "serde",
  "techscript_ast",
  "techscript_common",
@@ -3946,7 +4003,7 @@ dependencies = [
 name = "techscript_lsp"
 version = "2.0.0"
 dependencies = [
- "criterion",
+ "criterion 0.8.2",
  "serde",
  "serde_json",
  "techscript_ast",
diff --git a/compiler/ir/Cargo.toml b/compiler/ir/Cargo.toml
index 862a54bf..8bc0f91d 100644
--- a/compiler/ir/Cargo.toml
+++ b/compiler/ir/Cargo.toml
@@ -16,3 +16,8 @@ serde = { workspace = true }
 techscript_lexer = { path = "../lexer" }
 techscript_parser = { path = "../parser" }
 techscript_semantic = { path = "../semantic" }
+criterion = "0.5"
+
+[[bench]]
+name = "builder_bench"
+harness = false
diff --git a/compiler/ir/benches/builder_bench.rs b/compiler/ir/benches/builder_bench.rs
new file mode 100644
index 00000000..3a1c5864
--- /dev/null
+++ b/compiler/ir/benches/builder_bench.rs
@@ -0,0 +1,38 @@
+use criterion::{criterion_group, criterion_main, Criterion};
+use techscript_ir::builder::IRBuilder;
+use techscript_ir::types::IRType;
+
+fn criterion_benchmark(c: &mut Criterion) {
+    c.bench_function("seal_function 10000 blocks", |b| {
+        b.iter(|| {
+            let mut builder = IRBuilder::new();
+            builder.start_function("bench_func".to_string(), IRType::Void);
+
+            let num_blocks = 10000;
+            let mut blocks = vec![];
+            for i in 0..num_blocks {
+                let block_id = builder.new_block(format!("block_{}", i));
+                blocks.push(block_id);
+            }
+
+            for i in 0..(num_blocks - 1) {
+                builder.enter_block(blocks[i], format!("block_{}", i));
+                builder.emit_terminator(
+                    techscript_ir::instruction::TerminatorKind::Jump(blocks[i + 1]),
+                    techscript_common::Span { start: 0, end: 0 },
+                );
+            }
+
+            builder.enter_block(blocks[num_blocks - 1], format!("block_{}", num_blocks - 1));
+            builder.emit_terminator(
+                techscript_ir::instruction::TerminatorKind::Return(None),
+                techscript_common::Span { start: 0, end: 0 },
+            );
+
+            builder.seal_function();
+        });
+    });
+}
+
+criterion_group!(benches, criterion_benchmark);
+criterion_main!(benches);
diff --git a/compiler/ir/src/builder.rs b/compiler/ir/src/builder.rs
index b7ebaf52..8274ee9d 100644
--- a/compiler/ir/src/builder.rs
+++ b/compiler/ir/src/builder.rs
@@ -263,15 +263,15 @@ impl IRBuilder {
             }
 
             // Map predecessors
-            for i in 0..num_blocks {
-                let block_id = func.blocks[i].id;
-                let mut preds = Vec::new();
-                for other in &func.blocks {
-                    if other.successors.contains(&block_id) {
-                        preds.push(other.id);
-                    }
+            let mut pred_map = HashMap::new();
+            for block in &func.blocks {
+                for &succ in &block.successors {
+                    pred_map.entry(succ).or_insert_with(Vec::new).push(block.id);
                 }
-                func.blocks[i].predecessors = preds;
+            }
+
+            for block in &mut func.blocks {
+                block.predecessors = pred_map.remove(&block.id).unwrap_or_default();
             }
 
             self.functions.push(func);