diff --git a/crates/specialized/rtx-cfd/src/solvers/incompressible/poisson.rs b/crates/specialized/rtx-cfd/src/solvers/incompressible/poisson.rs
index 5798fb4..a545eed 100644
--- a/crates/specialized/rtx-cfd/src/solvers/incompressible/poisson.rs
+++ b/crates/specialized/rtx-cfd/src/solvers/incompressible/poisson.rs
@@ -604,6 +604,7 @@ impl<T: MgScalar> Level<T> {
         self.cells
             .par_iter()
             .zip(tmp[..n].par_iter_mut())
+            .with_min_len(PAR_MIN_LEN)
             .for_each(|(&idx, t)| {
                 *t = b[idx] - (self.ap[idx] * x[idx] - self.neighbour_sum(x, idx));
             });
@@ -628,6 +629,7 @@ impl<T: MgScalar> Level<T> {
             colour
                 .par_iter()
                 .zip(tmp[..n].par_iter_mut())
+                .with_min_len(PAR_MIN_LEN)
                 .for_each(|(&idx, t)| {
                     *t = (b[idx] + self.neighbour_sum(x_ro, idx)) / self.ap[idx];
                 });
@@ -802,10 +804,11 @@ impl<T: MgScalar> Hierarchy<T> {
                 wf.x[idx] = T::ZERO;
             }
             let Work { b, x, r, tmp } = wf;
+            let par_here = parallel && fine.cells.len() >= PAR_MIN_CELLS;
             for _ in 0..self.sweeps {
-                fine.smooth(b, x, tmp, smoother, parallel);
+                fine.smooth(b, x, tmp, smoother, par_here);
             }
-            if parallel {
+            if par_here {
                 fine.residual_par(b, x, r, tmp);
             } else {
                 fine.residual(b, x, r);
@@ -844,8 +847,9 @@ impl<T: MgScalar> Hierarchy<T> {
                 wf.x[idx] += T::from_f64(COARSE_CORRECTION) * wc.x[fine.coarse_of[idx]];
             }
             let Work { b, x, tmp, .. } = wf;
+            let par_here = parallel && fine.cells.len() >= PAR_MIN_CELLS;
             for _ in 0..self.sweeps {
-                fine.smooth(b, x, tmp, smoother, parallel);
+                fine.smooth(b, x, tmp, smoother, par_here);
             }
         }
         for &idx in &levels[0].cells {
@@ -976,6 +980,14 @@ impl<T: MgScalar> Prepared<T> {
     }
 }
 
+/// Threads pay only when a level is large: below this many active cells a
+/// level's maps run serially even with `threads > 1` (the fork-join of a
+/// map over a few thousand cells costs more than the map), and each task
+/// covers at least [`PAR_MIN_LEN`] cells. Both are pure scheduling: the
+/// values are the serial ones.
+const PAR_MIN_CELLS: usize = 8192;
+const PAR_MIN_LEN: usize = 2048;
+
 /// Configure rayon's global thread pool for the multigrid's parallel maps
 /// (PERF-2 P2). Idempotent: a pool that already exists is kept (rayon
 /// refuses a second global pool) — the first caller decides.