diff --git a/stdlib/dagger/dagger.cue b/stdlib/dagger/dagger.cue
index 2f66e734..39128169 100644
--- a/stdlib/dagger/dagger.cue
+++ b/stdlib/dagger/dagger.cue
@@ -1,14 +1,17 @@
 package dagger
 
 import (
+	"dagger.io/dagger/op"
 )
-// "dagger.io/dagger/op"
 
 // An artifact such as source code checkout, container image, binary archive...
 // May be passed as user input, or computed by a buildkit pipeline
 
-// FIXME (perf). See https://github.com/dagger/dagger/issues/445
-#Artifact: _
+#Artifact: {
+	#up: [...op.#Op]
+	_
+	...
+}
 
 // Secret value
 // FIXME: currently aliased as a string to mark secrets
diff --git a/stdlib/dagger/op/op.cue b/stdlib/dagger/op/op.cue
index b590160e..18f1e3ee 100644
--- a/stdlib/dagger/op/op.cue
+++ b/stdlib/dagger/op/op.cue
@@ -2,7 +2,16 @@
 package op
 
 // One operation in a pipeline
-#Op: #Export |
+//
+// #Op does not enforce the op spec at full resolution, to avoid triggering performance issues.
+// See https://github.com/dagger/dagger/issues/445
+#Op: {
+	do: string
+	...
+}
+
+// Full resolution schema enforciong the complete op spec
+#OpFull: #Export |
 	#FetchContainer |
 	#PushContainer |
 	#FetchGit |
@@ -48,10 +57,7 @@ package op
 	// `true` means also ignoring the mount cache volumes
 	always?: true | *false
 	dir:     string | *"/"
-	// FIXME (perf): complex schema in low-level ops causes explosive perf issues
-	//    see https://github.com/dagger/dagger/issues/445
-	// mount: [string]: "tmpfs" | "cache" | {from: _, path: string | *"/"}
-	mount: [string]: _
+	mount: [string]: "tmpfs" | "cache" | {from: _, path: string | *"/"}
 	// Map of hostnames to ip
 	hosts?: [string]: string
 	// User to exec with (if left empty, will default to the set user in the image)
diff --git a/stdlib/os/container.cue b/stdlib/os/container.cue
index 92fddc52..f13185f8 100644
--- a/stdlib/os/container.cue
+++ b/stdlib/os/container.cue
@@ -50,8 +50,7 @@ import (
 	// Mount is active when executing `command`, but not `setup`.
 
 	mount: [string]: {
-		// FIXME(perf) should be #Artifact. See https://github.com/dagger/dagger/issues/445
-		from: {...}
+		from: dagger.#Artifact
 		// FIXME: support source path
 	}