Need help with Armored Core Nexus

[VNgamer]

I'm playing fine until enemy Armored Cores appear then lags to near unplayable degree but the lag disappears as soon as enemy ACs are destroyed
Here's my specs:
I5 3470
8gb DDR3
AMD HD 6670 DDR3
Speed hack: Default/Recommended 
Resolution: Native
Renderer: DX11 (Hardware mode)