diff --git a/run.sh b/run.sh
index bddcc99..ac1b3d4 100755
--- a/run.sh
+++ b/run.sh
@@ -10,7 +10,7 @@
 # VM_ISO=<path> boots that ISO for `install` instead of cachyos.iso; with
 # VM_KERNEL/VM_INITRD/VM_APPEND its kernel is booted directly with those
 # parameters (scripts/vminstall.sh, the unattended install).
-# VM_CPUS=<n>: guest CPUs (6); ISO builds use all host cores but 2.
+# VM_CPUS=<n>: guest CPUs (6); ISO builds use ~75% of host cores (nproc * 3 / 4), so the host stays usable.
 # VM_PORT=<n>: the host port for the guest's SSH (2222), e.g. a second VM.
 # BIOS_VERSION=F7F0107 makes the guest report that BIOS version (DMI), e.g.
 # to test the wizard's BIOS update item; the firmware itself doesn't change.
diff --git a/scripts/vmisobuild.sh b/scripts/vmisobuild.sh
index 0752672..b3288a8 100755
--- a/scripts/vmisobuild.sh
+++ b/scripts/vmisobuild.sh
@@ -9,7 +9,7 @@
 #                 REPO) instead of the newest release
 # The VM must run with its cache shared (one per VM, next to its disk), and
 # gets all host cores but 2 for the build: start it with
-#   VM_CACHE=$VM_DIR/iso-cache VM_CPUS=$(( $(nproc) - 2 )) scripts/vmreset.sh --fremont
+#   VM_CACHE=$VM_DIR/iso-cache VM_CPUS=$(( $(nproc) * 3 / 4 )) scripts/vmreset.sh --fremont
 # Env: VM_DIR (see common.sh), ISO_BRANCH (feat/steamify), ISO_OUT (the host's
 #      steammachine-cachyos-live-iso/out/desktop).
 set -euo pipefail