@kYutobi it would if it's routed between internal subnets (and the MX is doing L3 routing between them). But I think what you're driving at is intra-subnet traffic?
(2 minutes go by...)
OK, just talked to my testing guy and we have done an RFC2544 test on the switching ability of the MX100 once, a long time ago. In that case, using intra-VLAN traffic the MX was able to switch traffic at or near line rate. But we have not ever done throughput tests routing between VLANs on the LAN ports.
So, knowing that firewall rules can be enforced I'm going to stick with my initial statement that any LAN traffic routed by the MX will be subject to the same inspection and WAN traffic, and will therefore hit the same throughput limits. But, if you're just switching traffic in the same VLAN between two MX ports it does sound like you can get near line rate out of the MX.