We did some performance and stability improvements to covers.openlibrary.org recently. Accessing book covers by ISBNs is quite faster now than before.
The covers.openlibrary.org website used to face intermittent internal errors because of memory leaks in the fastcgi library that was used. We switched to gunicorn server now and that solved this issue.
Please be aware that the OL covers API is intended for displaying book covers in other websites and not for bulk download. If you need to download book covers in bulk, please get in touch with us, we’ll be glad to help!