add setting for progressbar update period

javascript/progressbar.js

@@ -208,7 +208,7 @@ function requestProgress(id_task, progressbarContainer, gallery, atEnd, onProgre
 
             setTimeout(() => {
                 fun(id_task, res.id_live_preview);
-            }, 500)
+            }, opts.live_preview_refresh_period || 500)
         }, function(){
             removeProgressBar()
         })

modules/shared.py

@@ -455,6 +455,7 @@ options_templates.update(options_section(('ui', "Live previews"), {
     "show_progress_every_n_steps": OptionInfo(10, "Show new live preview image every N sampling steps. Set to -1 to show after completion of batch.", gr.Slider, {"minimum": -1, "maximum": 32, "step": 1}),
     "show_progress_type": OptionInfo("Approx NN", "Image creation progress preview mode", gr.Radio, {"choices": ["Full", "Approx NN", "Approx cheap"]}),
     "live_preview_content": OptionInfo("Prompt", "Live preview subject", gr.Radio, {"choices": ["Combined", "Prompt", "Negative prompt"]}),
+    "live_preview_refresh_period": OptionInfo(1000, "Progressbar/preview update period, in milliseconds")
 }))
 
 options_templates.update(options_section(('sampler-params', "Sampler parameters"), {