I tested chr 6.38.1 and x86 on the same server with virtual box. Virtual box setup :
Dual Xeon 56xx 32gb ram. (total of 16 cpu shown in MT)
Centos 7 running as host with latest intel drivers x520.
Paravirtualized drivers used
Virtualization acceleration used KVM
2 bridges one for lan and other for wan to pass vlans both having one interface in it on the host
Using it as a bras with 400 users chr started to show cpu usage to 50% and winbox got sluggish, Profiling showed cpu usage consumed by networking
On same platform same configurations with x86
2600 users running and no problem, CPU usage 60%
Unable to do tests on vmware workstation because it didnt supported vlans out of the box.
Any misconfigurations in VB to have such a poor performance