Limit number of threads.

cista/preview.py

@@ -92,7 +92,7 @@ def process_image(path, *, maxsize, quality):
     # Save as AVIF
     imgdata = io.BytesIO()
     t_save_start = perf_counter()
-    img.save(imgdata, format="avif", quality=quality, speed=10)
+    img.save(imgdata, format="avif", quality=quality, speed=10, max_threads=1)
     t_save_end = perf_counter()
 
     ret = imgdata.getvalue()