The internet speed when using a PCI Express network card on the ASUS Prime B450-Plus motherboard will not necessarily be capped at 1 Gbit, but it depends on the specific network card you use.

- The motherboard has three PCI Express 2.0 x1 slots, which can support network cards. However, these slots operate at PCIe 2.0 speeds, which is not the limiting factor for Gigabit Ethernet or even faster speeds since Ethernet speeds are typically limited by the Ethernet standard rather than the PCIe slot speed.
- If you use a Gigabit Ethernet network card, it will operate at up to 1 Gbps, which is the maximum speed of Gigabit Ethernet.
- If you use a faster network card, such as a 10 GbE card, it will still be limited by the PCIe 2.0 x1 slot's bandwidth, but this is usually not a bottleneck for Ethernet speeds up to 10 Gbps, as the network card's capabilities and the Ethernet standard itself are the primary limiting factors.

In summary, the internet speed will not be capped by the PCIe slot itself, but rather by the capabilities of the network card you install.