diff --git a/.cargo/config.toml b/.cargo/config.toml
new file mode 100644
index 0000000..45f5bff
--- /dev/null
+++ b/.cargo/config.toml
@@ -0,0 +1,7 @@
+# Test books (a few KB each, every journal append synced) go in the system
+# temp dir, /tmp, even when TMPDIR points elsewhere: `just mutants*` sets
+# TMPDIR to ~/.cache for cargo-mutants' multi-GB tree copies, and cargo
+# passes [env] to the tests it runs, not to cargo-mutants itself. On disk
+# the synced appends made each mutant's tests several times slower.
+[env]
+TMPDIR = { value = "/tmp", force = true }