Why is ethernet slower than wifi?

1. Your computer might only have 100-BaseTX Ethernet, whereas your WiFi might be 400mbps
2. Your cable may only be Cat5e which limits speeds, Cat6 is better

Cat5e supports 1G networks, so it's fast enough.

Not always, Cat5e can't be certified Gigabit for a reason. My Cat5e cable runs @ 650Mbps.

Nope, Cat5e is certified at 1000Mb/s. And the wireless will not even be close to that.

Hmm. My Cat5e cable never got close to Gigabit.

Not all cable is the same, especially the cheap stuff, and you may not exceed 100 meters length.