diff --git a/include/option.sh b/include/option.sh
index 3dc9db4..45b6f67 100755
--- a/include/option.sh
+++ b/include/option.sh
@@ -14,6 +14,14 @@ tmpgit="${PWD}/tmp/gitclone"
 
 eval "$(setvars "" board boarddir)"
 
+threads=
+set | grep CBMK_THREADS 1>/dev/null 2>/dev/null || threads=$(nproc) || :
+[ -z "$threads" ] && threads=$CBMK_THREADS
+[ -z "$threads" ] && threads=1 # CBMK_THREADS not set, and nproc failed
+
+expr "X$threads" : "X-\{0,1\}[0123456789][0123456789]*$" \
+    1>/dev/null 2>/dev/null || threads=1 # user specified a non-integer
+
 items()
 {
 	rval=1
diff --git a/script/update/release b/script/update/release
index 44d400f..5fccea6 100755
--- a/script/update/release
+++ b/script/update/release
@@ -192,11 +192,12 @@ mktarball()
 		mkdir -p "${2%/*}" || $err "mk, !mkdir -p \"${2%/*}\""
 	if [ "${tar_implementation% *}" = "tar (GNU tar)" ]; then
 		tar --sort=name --owner=root:0 --group=root:0 \
-		    --mtime="UTC 2023-11-06" -c "$1" | xz -T0 -9e > "$2" || \
-		    $err "mktarball 1, ${1}"
+		    --mtime="UTC 2023-11-06" -c "$1" | xz -T$threads -9e \
+		    > "$2" || $err "mktarball 1, ${1}"
 	else
 		# TODO: reproducible tarballs on non-GNU systems
-		tar -c "$1" | xz -T0 -9e > "$2" || $err "mktarball 2, $1"
+		tar -c "$1" | xz -T$threads -9e > "$2" || \
+		    $err "mktarball 2, $1"
 	fi
 	(
 	[ "${2%/*}" != "${2}" ] && x_ cd "${2%/*}"
diff --git a/script/update/trees b/script/update/trees
index a4d76ec..927f54a 100755
--- a/script/update/trees
+++ b/script/update/trees
@@ -169,7 +169,7 @@ check_cross_compiler()
 		[ -n "${xlang}" ] && export BUILD_LANGUAGES="$xlang"
 
 		[ -d "${cbdir}/util/crossgcc/xgcc/${_xarch}/" ] && continue
-		x_ make -C "$cbdir" crossgcc-${_xarch%-*} CPUS=$(nproc)
+		x_ make -C "$cbdir" crossgcc-${_xarch%-*} CPUS=$threads
 	done
 }
 
@@ -216,7 +216,7 @@ run_make_command()
 	[ "$project" = "coreboot" ] && [ -z "$mode" ] && x_ \
 		printf "%s\n" "${version%%-*}" > "$codedir/.coreboot-version"
 
-	make $mode -j$(nproc) $makeargs -C "$codedir" || \
+	make $mode -j$threads $makeargs -C "$codedir" || \
 	    $err "run_make $codedir: !make $mode"
 
 	[ "$mode" != "clean" ] && return 0