diff --git a/doc/build.md b/doc/build.md
index 157275318..cc93bb6cc 100644
--- a/doc/build.md
+++ b/doc/build.md
@@ -31,9 +31,11 @@ you can start mining by building and runing grin-miner against your running Grin
 ```sh
 git clone https://github.com/mimblewimble/grin.git
 cd grin
-cargo build
+cargo build --release
 ```
 
+Grin can also be built in debug mode (without the `--release` flag) but this will render fast sync prohibitively slow due to the large overhead of cryptographic operations.
+
 
 ### Cross-platform builds