adding details

Author: Fahnenfluchtige

docs/env_variables.md

@@ -115,7 +115,9 @@ fairly broad use of environment variables instead:
     - `AFL_USE_TSAN=1` - activates the thread sanitizer to find thread race
       conditions
     - `AFL_USE_UBSAN=1` - activates the undefined behavior sanitizer
-    - `AFL_UBSAN_VERBOSE=1` - outputs detailed diagnostic information when undefined behavior is detected, instead of simply terminating with "Illegal Instruction"   
+    - `AFL_UBSAN_VERBOSE=1` - outputs detailed diagnostic information when undefined behavior is detected, instead of simply terminating with "Illegal Instruction"
+
+    - Note: both `AFL_CFISAN_VERBOSE=1` and `AFL_UBSAN_VERBOSE=1` are disabled by default as verbose output can significantly slow down fuzzing performance. Use these options only during debugging or when additional crash diagnostics are required
 
   - `TMPDIR` is used by afl-as for temporary files; if this variable is not set,
     the tool defaults to /tmp.