Bulk Smush slows down and eventually hangs

Hi, when I run bulk smush against 3727 images, it will start at around 20s per image and slow down until after about 20 images, where it will seemingly stop altogether.

Any advice on how to avoid this? Any server configuration changes (VPS)? We are running WP Super Cache, is that likely to interfere? I notice we also get quite a few 'Error posting to API: 500 Internal Server Error' messages. Seems to affect around 10% of images.

Any help greatly appreciated.