Will this slow down the admin save action?

Not if you use the recommended queue-based wiring. The observer only publishes a message to RabbitMQ (~5ms); the actual image processing happens out-of-band in the worker. The admin save action stays well under 50ms of overhead. The HTTP-webhook variant (option 1 in the integration list) does block the save while the worker processes, that’s fine for tiny stores under 50 products / day, but at any scale you want the queue. If you measure the admin save action taking > 100ms extra after wiring, your observer is doing too much; pull more into the worker.