diff --git a/simulator/runner/file.rs b/simulator/runner/file.rs
index 435a60997..1f0ad838e 100644
--- a/simulator/runner/file.rs
+++ b/simulator/runner/file.rs
@@ -3,7 +3,7 @@ use std::{
     sync::Arc,
 };
 
-use limbo_core::{File, Result};
+use limbo_core::{CompletionType, File, Result};
 use rand::Rng as _;
 use rand_chacha::ChaCha8Rng;
 use tracing::{instrument, Level};
@@ -71,14 +71,11 @@ impl SimulatorFile {
     }
 
     #[instrument(skip_all, level = Level::TRACE)]
-    fn generate_latency(&self) {
+    fn generate_latency_duration(&self) -> Option<std::time::Duration> {
         let mut rng = self.rng.borrow_mut();
         // Chance to introduce some latency
-        if rng.gen_bool(self.latency_probability as f64 / 100.0) {
-            let latency = std::time::Duration::from_millis(rng.gen_range(20..50));
-            tracing::trace!(?latency);
-            std::thread::sleep(latency);
-        }
+        rng.gen_bool(self.latency_probability as f64 / 100.0)
+            .then(|| std::time::Duration::from_millis(rng.gen_range(20..50)))
     }
 }
 
@@ -89,7 +86,6 @@ impl File for SimulatorFile {
                 "Injected fault".into(),
             ));
         }
-        self.generate_latency();
         self.inner.lock_file(exclusive)
     }
 
@@ -99,11 +95,14 @@ impl File for SimulatorFile {
                 "Injected fault".into(),
             ));
         }
-        self.generate_latency();
         self.inner.unlock_file()
     }
 
-    fn pread(&self, pos: usize, c: Arc<limbo_core::Completion>) -> Result<()> {
+    fn pread(
+        &self,
+        pos: usize,
+        mut c: limbo_core::Completion,
+    ) -> Result<Arc<limbo_core::Completion>> {
         self.nr_pread_calls.set(self.nr_pread_calls.get() + 1);
         if self.fault.get() {
             self.nr_pread_faults.set(self.nr_pread_faults.get() + 1);
@@ -111,7 +110,24 @@ impl File for SimulatorFile {
                 "Injected fault".into(),
             ));
         }
-        self.generate_latency();
+        if let Some(latency) = self.generate_latency_duration() {
+            let CompletionType::Read(read_completion) = &mut c.completion_type else {
+                unreachable!();
+            };
+            let before = self.rng.borrow_mut().gen_bool(0.5);
+            let dummy_complete = Box::new(|_| {});
+            let prev_complete = std::mem::replace(&mut read_completion.complete, dummy_complete);
+            let new_complete = move |res| {
+                if before {
+                    std::thread::sleep(latency);
+                }
+                (prev_complete)(res);
+                if !before {
+                    std::thread::sleep(latency);
+                }
+            };
+            read_completion.complete = Box::new(new_complete);
+        };
         self.inner.pread(pos, c)
     }
 
@@ -119,8 +135,8 @@ impl File for SimulatorFile {
         &self,
         pos: usize,
         buffer: Arc<RefCell<limbo_core::Buffer>>,
-        c: Arc<limbo_core::Completion>,
-    ) -> Result<()> {
+        mut c: limbo_core::Completion,
+    ) -> Result<Arc<limbo_core::Completion>> {
         self.nr_pwrite_calls.set(self.nr_pwrite_calls.get() + 1);
         if self.fault.get() {
             self.nr_pwrite_faults.set(self.nr_pwrite_faults.get() + 1);
@@ -128,13 +144,47 @@ impl File for SimulatorFile {
                 "Injected fault".into(),
             ));
         }
-        self.generate_latency();
+        if let Some(latency) = self.generate_latency_duration() {
+            let CompletionType::Write(write_completion) = &mut c.completion_type else {
+                unreachable!();
+            };
+            let before = self.rng.borrow_mut().gen_bool(0.5);
+            let dummy_complete = Box::new(|_| {});
+            let prev_complete = std::mem::replace(&mut write_completion.complete, dummy_complete);
+            let new_complete = move |res| {
+                if before {
+                    std::thread::sleep(latency);
+                }
+                (prev_complete)(res);
+                if !before {
+                    std::thread::sleep(latency);
+                }
+            };
+            write_completion.complete = Box::new(new_complete);
+        };
         self.inner.pwrite(pos, buffer, c)
     }
 
-    fn sync(&self, c: Arc<limbo_core::Completion>) -> Result<()> {
+    fn sync(&self, mut c: limbo_core::Completion) -> Result<Arc<limbo_core::Completion>> {
         self.nr_sync_calls.set(self.nr_sync_calls.get() + 1);
-        self.generate_latency();
+        if let Some(latency) = self.generate_latency_duration() {
+            let CompletionType::Sync(sync_completion) = &mut c.completion_type else {
+                unreachable!();
+            };
+            let before = self.rng.borrow_mut().gen_bool(0.5);
+            let dummy_complete = Box::new(|_| {});
+            let prev_complete = std::mem::replace(&mut sync_completion.complete, dummy_complete);
+            let new_complete = move |res| {
+                if before {
+                    std::thread::sleep(latency);
+                }
+                (prev_complete)(res);
+                if !before {
+                    std::thread::sleep(latency);
+                }
+            };
+            sync_completion.complete = Box::new(new_complete);
+        };
         self.inner.sync(c)
     }
 
@@ -148,3 +198,5 @@ impl Drop for SimulatorFile {
         self.inner.unlock_file().expect("Failed to unlock file");
     }
 }
+
+struct Latency {}