Ethernet is not necessarily realtime. You can have very lengthy delays unless you are planning a one-on-one connection with no possibility of collisions, retransmissions, etc.

While I've been retired for 20+ years, I've been in a few thousand manufacturing facilities. (I ran the US subsidiary of a French machine tool builder.)

Are you wanting to control a CNC machine from a PC? Is the PIC going to be on the CNC machine or PC end of the link?