diff --git a/docs/CHANGELOG.md b/docs/CHANGELOG.md
index 8ec48905..014a35e6 100644
--- a/docs/CHANGELOG.md
+++ b/docs/CHANGELOG.md
@@ -27,6 +27,10 @@ This is an UNSTABLE release. It includes many experimental features, as well as
 * `images_per_file` is chosen from the acquisition when it is not given: a rotation sweep of at most 20000 images goes into a single data file, a grid scan splits on whole fast-axis rows, and stills and serial keep 1000.
 * rugnux: observations outside the scaling resolution range are dropped as they are ingested, rather than scaled, combined and error-modelled first; merged results are unchanged.
 * rugnux: two runs of the same command on the same images now produce the same intensities - the GPU prediction, profile-learning and profile-fit steps no longer depend on the order their blocks happen to run in.
+* rugnux: the offline lattice refinement is bounded by iterations rather than by a wall clock, so a loaded machine can no longer refine to a different lattice; a live acquisition keeps its real-time bound.
+* rugnux: a rotation run is substantially faster - a 24-crystal set that took 7m55s takes 6m28s, with the space group unchanged on every one of them. Most of it is work that was being done and thrown away: the geometry pre-pass no longer merges, corrects, and writes a result the second pass replaces, observations outside the scaling resolution range are never built, and the detector lookup tables are built once for the run rather than once per worker.
+* rugnux: the geometry pre-pass no longer writes `<prefix>_01.mtz`, `_01.cif`, `_01.hkl` or its scaling table. The second pass rewrites all of it at the refined geometry seconds later, and that is the result to use.
+* rugnux: with `-N` left at its default the per-image loop uses at most eight workers per GPU. Past that, workers spend their time queueing kernels rather than running them - measured, a 16 Mpx set is slower at 48 workers than at 8. An explicit `-N` is obeyed as given.
 * rugnux: the detector-frame modulation correction is fitted on a grid spanning the detector rather than on the reflections that happen to be present, so whether it is applied no longer depends on how far integration reached.
 * rugnux: the first-pass rotation indexing finds its spots on every worker rather than one, which is worth most on large detectors. The lattice it picks is unchanged.
 * rugnux: beam-stop detection is substantially faster - the pre-scan reads and accumulates its frames on several threads, the mask is built in one pass over the image rather than by repeated searches, and a worker's accumulator is allocated only if it is used. The shadow it finds is unchanged.
diff --git a/rugnux/Rugnux.cpp b/rugnux/Rugnux.cpp
index a8465297..f11dc86c 100644
--- a/rugnux/Rugnux.cpp
+++ b/rugnux/Rugnux.cpp
@@ -1826,10 +1826,24 @@ ProcessResult Rugnux::RunPipeline(RugnuxObserver *observer, bool write_output, b
 
     std::function<void()> worker = per_image_analysis ? std::function<void()>(full_worker)
                                                       : std::function<void()>(azint_worker);
+    // How many workers the loop actually wants. Every one of them submits its own kernels to a card,
+    // and a card runs out of room to accept them long before it runs out of work: measured on two
+    // GPUs, the loop's own time falls to sixteen workers and then rises again, so forty-eight is
+    // slower than eight on a 16 Mpx set. The cap is per card because that is what the queue depth
+    // belongs to, and it only applies when -N was left at its default - an explicit -N is a
+    // deliberate instruction and is obeyed, which is what a previous attempt at this got wrong.
+    const int gpus = get_gpu_count();
+    const int image_workers = (config_.nthreads_auto && gpus > 0)
+            ? std::min(config_.nthreads, std::max(8, 8 * gpus))
+            : config_.nthreads;
+    if (image_workers < config_.nthreads)
+        logger.Info("Per-image loop: {} of {} threads ({} GPUs) - more workers than a card can take "
+                    "queue work from make it slower, not faster; pass -N to override",
+                    image_workers, config_.nthreads, gpus);
     std::vector<std::future<void> > futures;
-    futures.reserve(config_.nthreads);
+    futures.reserve(image_workers);
     const auto image_loop_start = std::chrono::steady_clock::now();
-    for (int i = 0; i < config_.nthreads; ++i)
+    for (int i = 0; i < image_workers; ++i)
         futures.push_back(std::async(std::launch::async, worker));
     for (auto &f: futures)
         f.get();
diff --git a/rugnux/Rugnux.h b/rugnux/Rugnux.h
index bf7fd54c..deb9b07a 100644
--- a/rugnux/Rugnux.h
+++ b/rugnux/Rugnux.h
@@ -44,6 +44,9 @@ struct ProcessConfig {
     int end_image = -1;     // -1 => to the end of the dataset
     int stride = 1;
     int nthreads = 1;
+    // True when -N was left at its default and nthreads above is just the machine's thread count.
+    // The per-image loop is capped in that case (see RunPipeline); an explicit -N never is.
+    bool nthreads_auto = false;
 
     // Output prefix for the _process.h5 (and scaled reflections). Empty => process without writing.
     std::string output_prefix;
diff --git a/rugnux/rugnux_cli.cpp b/rugnux/rugnux_cli.cpp
index fae3e0a6..4a0032a8 100644
--- a/rugnux/rugnux_cli.cpp
+++ b/rugnux/rugnux_cli.cpp
@@ -1187,6 +1187,7 @@ static int RunRugnux(int argc, char **argv) {
     // behaves the same. The scale/merge engines expand 0 on their own, but the per-image processing
     // loop (Rugnux) spawns exactly nthreads workers, so passing 0 there would spawn none and process
     // nothing - hence resolving it centrally rather than relying on each consumer.
+    const bool nthreads_auto = nthreads <= 0;
     if (nthreads <= 0) {
         unsigned int hw = std::thread::hardware_concurrency();
         nthreads = hw > 0 ? static_cast<int>(hw) : 1;
@@ -1642,6 +1643,7 @@ static int RunRugnux(int argc, char **argv) {
         config.end_image = end_image;
         config.stride = image_stride;
         config.nthreads = nthreads;
+    config.nthreads_auto = nthreads_auto;
         config.output_prefix = output_prefix;
         config.detect_beam_stop = detect_beam_stop;
         config.estimate_beam_center = estimate_beam_center;
@@ -1686,6 +1688,7 @@ static int RunRugnux(int argc, char **argv) {
         config.end_image = end_image;
         config.stride = image_stride;
         config.nthreads = nthreads;
+    config.nthreads_auto = nthreads_auto;
         config.output_prefix = output_prefix;
         config.detect_beam_stop = detect_beam_stop;
         config.estimate_beam_center = estimate_beam_center;
@@ -2089,6 +2092,7 @@ static int RunRugnux(int argc, char **argv) {
     config.end_image = end_image;
     config.stride = image_stride;
     config.nthreads = nthreads;
+    config.nthreads_auto = nthreads_auto;
     config.output_prefix = output_prefix;
     config.spot_finding = spot_settings;
     config.rotation_indexing = rotation_indexing;