diff --git a/docs/known-issues.md b/docs/known-issues.md
index 00dd7fb..b0d534c 100644
--- a/docs/known-issues.md
+++ b/docs/known-issues.md
@@ -171,6 +171,11 @@ scripts for this are worth keeping; note that wrapping them in `timeout` SIGTERM
   2026-10-14 (docs/tower-installation.md, Phase 2b). Not yet tested: a pulled camera cable during a scan
   (the reconnect is only unit-tested), and a long unattended run. A NIR unit would be a third driver
   behind the same key.
+- The InfraTec SDK costs about a quarter of one core on the 3 Delta even between pictures: three threads
+  it starts in `dll_init` and one per connection poll the raw socket in `nanosleep` loops (measured on
+  tower 111, 2026-10-14). The frame rate does not change it. In the first minutes after deploying, the
+  CPU guard slowed the scan once (86 °C, back to normal after 2 min). Watch Health `cpu_c` over a warm
+  day before a tower without air flow gets a thermal camera.
 - Tower 111's colour profile does not apply: every start since 2026-10-07 logs `applying profile 'standard'
   from .../rgb/standard.xml failed (error -39)`, and the camera runs on whatever it had. Unrelated to the
   thermal camera; seen again while testing it.