Roll back to stock firmware, archer c7

[Lantis]

This is detailed several times, so i'm going to repeat it briefly.

Qos and Bwmon take up CPU resource.
Your router has a slow CPU.
Your performance will be limited as a result.

Openwrt does not include the same bandwidth monitoring or QOS implementation, nor are they enabled by default. This accounts for the faster speed.

Additionally, Gargoyle is built for many devices at the same time. The build you have linked has been specifically optimised for that one device. Of course it is going to perform better.
If you want to do a custom build of Gargoyle including this, by all means do so.