diff --git a/crates/mun/src/main.rs b/crates/mun/src/main.rs
index 989d2928c..398797796 100644
--- a/crates/mun/src/main.rs
+++ b/crates/mun/src/main.rs
@@ -4,7 +4,7 @@ extern crate failure;
 use std::time::Duration;
 
 use clap::{App, AppSettings, Arg, ArgMatches, SubCommand};
-use mun_runtime::invoke_fn;
+use mun_runtime::{invoke_fn, MunRuntime, RuntimeBuilder};
 
 fn main() -> Result<(), failure::Error> {
     let matches = App::new("mun")
@@ -56,9 +56,8 @@ fn main() -> Result<(), failure::Error> {
                 .arg(
                     Arg::with_name("delay")
                         .long("delay")
-                        .required(true)
                         .takes_value(true)
-                        .help("how much to delay received filesystem events (in ms). This allows bundling of identical events, e.g. when several writes to the same file are detected. A high delay will make hot reloading less responsive."),
+                        .help("how much to delay received filesystem events (in ms). This allows bundling of identical events, e.g. when several writes to the same file are detected. A high delay will make hot reloading less responsive. (defaults to 10 ms)"),
                 ),
         )
         .get_matches();
@@ -84,10 +83,11 @@ fn build(matches: &ArgMatches) -> Result<(), failure::Error> {
 
 /// Starts the runtime with the specified library and invokes function `entry`.
 fn start(matches: &ArgMatches) -> Result<(), failure::Error> {
-    let runtime_options = runtime_options(matches)?;
-    let mut runtime = mun_runtime::MunRuntime::new(runtime_options)?;
+    let mut runtime = runtime(matches)?;
 
     let entry_point = matches.value_of("entry").unwrap_or("main");
+
+    #[allow(clippy::unit_arg)]
     Ok(invoke_fn!(runtime, entry_point))
 }
 
@@ -107,11 +107,15 @@ fn compiler_options(matches: &ArgMatches) -> Result<mun_compiler::CompilerOption
     })
 }
 
-fn runtime_options(matches: &ArgMatches) -> Result<mun_runtime::RuntimeOptions, failure::Error> {
-    let delay: u64 = matches.value_of("delay").unwrap().parse()?;
+fn runtime(matches: &ArgMatches) -> Result<MunRuntime, failure::Error> {
+    let mut builder = RuntimeBuilder::new(
+        matches.value_of("LIBRARY").unwrap(), // Safe because its a required arg
+    );
 
-    Ok(mun_runtime::RuntimeOptions {
-        library_path: matches.value_of("LIBRARY").unwrap().into(), // Safe because its a required arg
-        delay: Duration::from_millis(delay),
-    })
+    if let Some(delay) = matches.value_of("delay") {
+        let delay: u64 = delay.parse()?;
+        builder.set_delay(Duration::from_millis(delay));
+    }
+
+    builder.spawn()
 }
diff --git a/crates/mun_runtime/src/lib.rs b/crates/mun_runtime/src/lib.rs
index 906a82904..043863aac 100644
--- a/crates/mun_runtime/src/lib.rs
+++ b/crates/mun_runtime/src/lib.rs
@@ -22,6 +22,31 @@ pub struct RuntimeOptions {
     pub delay: Duration,
 }
 
+/// A builder for the runtime.
+pub struct RuntimeBuilder {
+    options: RuntimeOptions,
+}
+
+impl RuntimeBuilder {
+    pub fn new<P: Into<PathBuf>>(library_path: P) -> Self {
+        Self {
+            options: RuntimeOptions {
+                library_path: library_path.into(),
+                delay: Duration::from_millis(10),
+            },
+        }
+    }
+
+    pub fn set_delay(&mut self, delay: Duration) -> &mut Self {
+        self.options.delay = delay;
+        self
+    }
+
+    pub fn spawn(self) -> Result<MunRuntime, Error> {
+        MunRuntime::new(self.options)
+    }
+}
+
 /// A runtime for the Mun scripting language.
 pub struct MunRuntime {
     assemblies: HashMap<PathBuf, Assembly>,
@@ -135,7 +160,7 @@ invoke_fn_impl! {
 
 #[cfg(all(test, windows))]
 mod tests {
-    use super::{invoke_fn, MunRuntime, RuntimeOptions};
+    use super::{invoke_fn, MunRuntime, RuntimeBuilder};
     use std::path::PathBuf;
     use std::time::Duration;
 
@@ -148,22 +173,14 @@ mod tests {
 
     #[test]
     fn mun_new_runtime() {
-        let options = RuntimeOptions {
-            library_path: test_lib_path(),
-            delay: Duration::from_millis(10),
-        };
-
-        let _runtime = MunRuntime::new(options).expect("Failed to initialize Mun runtime.");
+        let builder = RuntimeBuilder::new(test_lib_path());
+        let _runtime = builder.spawn().expect("Failed to initialize Mun runtime.");
     }
 
     #[test]
     fn mun_invoke_fn() {
-        let options = RuntimeOptions {
-            library_path: test_lib_path(),
-            delay: Duration::from_millis(10),
-        };
-
-        let mut runtime = MunRuntime::new(options).expect("Failed to initialize Mun runtime.");
+        let builder = RuntimeBuilder::new(test_lib_path());
+        let mut runtime = builder.spawn().expect("Failed to initialize Mun runtime.");
 
         let a: f64 = 4.0;
         let b: f64 = 2.0;