Faster network
Thanks to the new LAN7515 chip, Gigabit Ethernet is finally supported. However, you cannot expect the full gigabit data rate. The reason for this is that the chip hangs internally on a USB 2.0 controller, so 315 Mbit/s is the end. In its predecessor, the Raspberry Pi 3B, the speed was only 95 Mbps. Thus one can be completely content with the increase.
Higher CPU clock
The four ARM cores have also been improved. These now clock at 1.4 GHz instead of 1.2 GHz and are therefore around 17% faster. A sheet metal lid serves as a heat spreader to better dissipate the waste heat associated with the increased clock frequency. However, the higher clock speed also results in higher power consumption.
5 GHz WLAN ac
The Broadcom BCM43455 is a new WLAN controller. This now supports dual-band ac WLAN for the 5 GHz frequency band. Instead of two, however, only one antenna is installed, so that the throughput increases only moderately.
Power-over-Ethernet (prepared)
The Raspberry Pi 3B+ now has 4 pins for PoE on the board. A PoE HAT for the Raspberry Pi, which can supply the Raspi with up to 12.95 watts (IEEE 802.af), will be available soon.