Update docs/architecture/psa-thread-safety/psa-thread-safety.md
Co-authored-by: Paul Elliott <62069445+paul-elliott-arm@users.noreply.github.com>
Signed-off-by: Ryan Everett <ryan.everett@arm.com>
diff --git a/docs/architecture/psa-thread-safety/psa-thread-safety.md b/docs/architecture/psa-thread-safety/psa-thread-safety.md
index d33e0a6..edb94c5 100644
--- a/docs/architecture/psa-thread-safety/psa-thread-safety.md
+++ b/docs/architecture/psa-thread-safety/psa-thread-safety.md
@@ -147,6 +147,8 @@
A deadlock would occur if a thread attempts to lock a mutex while already holding it. Functions which need to be called while holding the global mutex have documentation to say this.
+To avoid performance degradation, functions must hold mutexes for as short a time as possible. In particular, they must not start expensive operations (eg. doing cryptography) while holding the mutex.
+
#### Key slots