diff --git a/libs/ardour/audioregion.cc b/libs/ardour/audioregion.cc
index 90364e13ac..80ba4d405b 100644
--- a/libs/ardour/audioregion.cc
+++ b/libs/ardour/audioregion.cc
@@ -2652,6 +2652,12 @@ AudioRegion::apply_region_fx (BufferSet& bufs, samplepos_t start_sample, samplep
 		return;
 	}
 
+	ProcessThread* pt = 0;
+	if (!ProcessThread::have_thread_buffers ()) {
+		pt = new ProcessThread ();
+		pt->get_buffers ();
+	}
+
 	pframes_t block_size = _session.get_block_size ();
 	if (_fx_block_size != block_size) {
 		_fx_block_size = block_size;
@@ -2677,7 +2683,7 @@ AudioRegion::apply_region_fx (BufferSet& bufs, samplepos_t start_sample, samplep
 				lm.release ();
 				/* this triggers a re-read */
 				const_cast<AudioRegion*>(this)->remove_plugin (rfx);
-				return;
+				goto out;
 			}
 			remain -= run;
 			offset += run;
@@ -2698,4 +2704,10 @@ AudioRegion::apply_region_fx (BufferSet& bufs, samplepos_t start_sample, samplep
 	}
 	_fx_pos = end_sample;
 	_fx_latent_read = false;
+
+out:
+	if (pt) {
+		pt->drop_buffers ();
+		delete pt;
+	}
 }
diff --git a/libs/ardour/globals.cc b/libs/ardour/globals.cc
index 4c0abd5720..65af635a14 100644
--- a/libs/ardour/globals.cc
+++ b/libs/ardour/globals.cc
@@ -742,9 +742,12 @@ ARDOUR::init (bool try_optimization, const char* localedir, bool with_gui)
 	 * each I/O thread (up to hardware_concurrency) and one for itself
 	 * (butler's main thread).
 	 *
-	 * In theory (2 * hw + 4) should be sufficient, let's add one for luck.
+	 * In theory (2 * hw + 4) should be sufficient, were it not for
+	 * AudioPlaylistSource and AudioRegionEditor::peak_amplitude_thread(s).
+	 * WaveViewThreads::start_threads adds `min (8, hw - 1)`
+	 *
 	 */
-	BufferManager::init (hardware_concurrency () * 2 + 5);
+	BufferManager::init (hardware_concurrency () * 3 + 6);
 
 	PannerManager::instance ().discover_panners ();
 
diff --git a/libs/ardour/playlist.cc b/libs/ardour/playlist.cc
index 517a0e9296..e5ca961e67 100644
--- a/libs/ardour/playlist.cc
+++ b/libs/ardour/playlist.cc
@@ -3190,9 +3190,15 @@ Playlist::combine (const RegionList& rl, std::shared_ptr<Track>)
 	pair<timepos_t,timepos_t> extent = pl->get_extent();
 	timepos_t zero (_type == DataType::AUDIO ? Temporal::AudioTime : Temporal::BeatTime);
 
+	/* This may require regionFx processing, via AudioRegion::read_at */
+	ARDOUR::ProcessThread* pt = new ProcessThread ();
+	pt->get_buffers ();
 	for (uint32_t chn = 0; chn < channels; ++chn) {
 		sources.push_back (SourceFactory::createFromPlaylist (_type, _session, pl, id(), parent_name, chn, zero, extent.second, false, false));
 	}
+	pt->drop_buffers ();
+	delete pt;
+
 
 	/* now a new whole-file region using the list of sources */