adjust generate previews

lib/templates/process_document.rb

@@ -90,8 +90,10 @@ module Templates
 
       pool = Concurrent::FixedThreadPool.new(concurrency)
 
+      blobs =
+        range.each_slice(concurrency).flat_map do |page_numbers|
           promises =
-        range.map do |page_number|
+            page_numbers.map do |page_number|
               doc_page = doc.get_page(page_number)
 
               hide_placeholder_widgets(doc_page, hide_empty: !flatten_pages)
@@ -106,7 +108,10 @@ module Templates
               doc_page&.close
             end
 
-      Concurrent::Promise.zip(*promises).value!.each do |blob|
+          Concurrent::Promise.zip(*promises).value!
+        end
+
+      blobs.each do |blob|
         next unless blob
 
         ApplicationRecord.no_touching do