diff --git a/ccp4/README.md b/ccp4/README.md
index 2887e3e..9020c85 100644
--- a/ccp4/README.md
+++ b/ccp4/README.md
@@ -43,6 +43,20 @@ https://www.ccp4.ac.uk/download/ and update `files/config.yaml`, or drop a
 hand-downloaded tarball into the distfiles dir under the `name:` given in
 `files/config.yaml`.
 
+## Install is SLOW — not stuck
+
+The `install` step untars 3.7 GB into ~15-20 GB of small files on shared
+/opt/psi storage: the tar alone runs 10-30 min with no output (gzip is
+single-threaded, small-file creation on network storage is the
+bottleneck), and `BINARY.setup` afterwards walks the whole installation
+for several more minutes. Watch progress from a second shell:
+
+```bash
+watch -n 30 'du -sh /opt/psi/MX/ccp4/9.0/'
+```
+
+Growing `du` = working. Only investigate if it's frozen for 10+ min.
+
 ## Known failure modes
 
 - **"done" but broken install**: modbuild has no errexit in hooks — errors in