yes the speed of the interface does not change, but the throughput could change due to additional frame overhead introduced - 32 bytes. You may want to set your interface MTU higher on MACSEC enabled interfaces. Of course I know of very few Ethernet links that are 100% utilized.
Not sure why you are implimenting MACSEC, especially considering the questions you are asking. Have you looked at:
https://www.juniper.net/techpubs/en_US/junos16.1/topics/concept/macsec.html
Here you will see no SW listing for EX4300 at this time.