Thumbnails taking too long to load

I am trying to load thumbnail using Glide but it is taking too long. The first few thumbnails load instantly but you have to wait quite a while for rest of them to load.

Glide.with(Context)
     .load(new File(videoFiles.get(position).getPath()))
     .placeholder(android.R.drawable.alert_dark_frame)
     .error(android.R.drawable.stat_notify_error)
     .into(holder.thumbnail);

I have also tried Picasso but the result is not much different.

Picasso.get()
                .load(new File(videoFiles.get(position).getPath()))
                .placeholder(alert_dark_frame)
                .error(stat_notify_error)
                .into(holder.thumbnail);

what implementation should I change to load them faster as expected?