diff --git a/plugins/csharp/Cargo.toml b/plugins/csharp/Cargo.toml
index baefcf7ecf2..d21cd000153 100644
--- a/plugins/csharp/Cargo.toml
+++ b/plugins/csharp/Cargo.toml
@@ -18,5 +18,6 @@ walkdir = "2"
 zip = "0.5"
 
 [dev-dependencies]
+once_cell = "1"
 simple_logger = "1"
 tempfile = "3"
diff --git a/plugins/csharp/src/plugin.rs b/plugins/csharp/src/plugin.rs
index bb36316bedb..9bf70b84938 100644
--- a/plugins/csharp/src/plugin.rs
+++ b/plugins/csharp/src/plugin.rs
@@ -350,10 +350,13 @@ impl LanguagePlugin for CSharpPlugin {
 #[cfg(test)]
 mod test {
     use super::*;
-    use std::sync::Once;
+    use once_cell::sync::Lazy;
+    use std::sync::{Mutex, Once};
     use tempfile::TempDir;
 
     static INIT_RUNNER: Once = Once::new();
+    // running the runner in parallel seems to sometimes make tests run for an excessively long time
+    static MUTEX: Lazy<Mutex<()>> = Lazy::new(|| Mutex::new(()));
 
     fn init() {
         use log::*;
@@ -524,6 +527,7 @@ mod test {
     #[test]
     fn scans_exercise() {
         init();
+        let _lock = MUTEX.lock().unwrap();
 
         let temp = dir_to_temp("tests/data/passing-exercise");
         let plugin = CSharpPlugin::new();
@@ -537,6 +541,7 @@ mod test {
     #[test]
     fn runs_tests_passing() {
         init();
+        let _lock = MUTEX.lock().unwrap();
 
         let temp = dir_to_temp("tests/data/passing-exercise");
         let plugin = CSharpPlugin::new();
@@ -553,6 +558,7 @@ mod test {
     #[test]
     fn runs_tests_failing() {
         init();
+        let _lock = MUTEX.lock().unwrap();
 
         let temp = dir_to_temp("tests/data/failing-exercise");
         let plugin = CSharpPlugin::new();
@@ -571,6 +577,7 @@ mod test {
     #[test]
     fn runs_tests_compile_err() {
         init();
+        let _lock = MUTEX.lock().unwrap();
 
         let temp = dir_to_temp("tests/data/non-compiling-exercise");
         let plugin = CSharpPlugin::new();
@@ -588,6 +595,7 @@ mod test {
     #[test]
     fn runs_tests_timeout() {
         init();
+        let _lock = MUTEX.lock().unwrap();
 
         let temp = dir_to_temp("tests/data/passing-exercise");
         let plugin = CSharpPlugin::new();
@@ -600,6 +608,7 @@ mod test {
     #[test]
     fn cleans() {
         init();
+        let _lock = MUTEX.lock().unwrap();
 
         let temp = dir_to_temp("tests/data/passing-exercise");
         let plugin = CSharpPlugin::new();
@@ -637,6 +646,7 @@ mod test {
     #[test]
     fn doesnt_give_points_unless_all_relevant_exercises_pass() {
         init();
+        let _lock = MUTEX.lock().unwrap();
 
         let temp = dir_to_temp("tests/data/partially-passing");
         let plugin = CSharpPlugin::new();