Since I don't have enough money yet to build a PC and it will take me another year of saving money, I really want to use an EGPU instead. But how bad the bottleneck will be and will it worth buying a Zotac GTX 1060 6GB Mini to pair with my laptop? If they can perform quite well then should I buy an external monitor?
PCIe 2.0 x1 is equivalent to PCIe 1.1 x2. While I don't see any PCIe scaling test for the 1060 it is roughly equivalent to a 980: None of those are Unreal Engine 4 games but you can see the trend. If it continues to PCIe 1.1 x2 you should see around 63% of the performance it would have at the PCIe 3.0 x16 it was designed for. That would still be twice as fast as the minimum recommended GTX 660.