There's not really any reason it would be slower. Being on-package doesn't offer any real performance advantage (the lengths of the traces to memory have a negligible impact on performance or latency), and is more of a benefit to manufacturing.

The memory on the M1 actually has fairly poor latency compared to most x86 systems:

https://www.anandtech.com/show/16252/mac-mini-apple-m1-teste...