So I've set up several VLANs on my 2860, here's some stats:
A file transfer between 2 devices on the same VLAN, will achieve a speed of 793Mbps TX / 931Mbps RX and CPU usage on the router does not exceed 5%
A file transfer between 2 devices on different VLANs (with route defined on the 2860) will achieve 351Mbps TX / 383Mbps RX with CPU usage of ~90%

In all cases the test was initiated on my laptop - VLAN1 to a NAS on either VLAN1 or VLAN100 depending on the test performed. I just moved the NAS into a different VLAN to keep the variables of the test to a minimum.

I suspect it's down to the CPU of the router having to work out which packet goes to which VLAN, so I'm just curious. Are these results expected ?

EDIT: For consistency, I performed the test using LAN speed test lite ( http://www.totusoft.com/lanspeed1 ) with a file size of 4096MB

Which firmware version is on the router? Also is Hardware Acceleration enabled on the router (set to Auto)?

The 2860ac's firewall throughput is quoted as being 200mbps and VLAN to VLAN transfers do require some processing by the firewall. I think Hardware Acceleration can increase that significantly though. Also, I recommend testing with the Vector 4 modem code instead of the BT firmware, to see if that affects your transfer speeds significantly.

Cheers for the reply.
I'm currently on 3.8.4.6_BT

I did perform the same tests at the time with HW enabled, the results where 350Mbps TX / 384Mbps RX so not really any difference. Within the margin of error I'd say.

Will give the vector 4 FW a try though and report back.

Results of the vector 4 test (3.8.4.6_VT4)
372Mbps TX / 394Mbps RX
371Mbps TX / 399Mbps RX with HW enabled (auto)

So the vector 4 firmware has made an improvement, not by a great deal, admittedly. The HW enabled/disabled doesn't seem to make any real difference. If anything, it seems to make it slightly worse.