Slow upload , but with proxy enabled it is fast

I’m fighting with this issue since the first day I have MT router ( ROS 4.x) . I have multiple mipsbe routers and all of them shows same behaviour. The first version which shows significant improvements is ROS 6.0rc5.
I have 100/100Mbps dedicated ports on a Cisco switches in a DC. Without MT router 100/100 is not an issue to anywhere in EU, but with MT router local sites are running close to 100/100, but distant sites are running 100Mbps down /15Mbps Up. Which is very very sad. This looks like some kind of src-NAT-ing / connection tracking / packet queueing issue.
With 6.0rc5 I have 100/~30Mbps, but if I upgrade to 6.0rc6 it’s back to old “good” 100/15. That’s sucks. Web-Proxy is not a solution for me due to multiple reasons. Any creative advice are welcome. I’m considering to purchase CCR router, but if that one has same issue, I will laugh an go with a basic Cisco ASA for “slightly” more money… :frowning: