I pay for a 150Mbps service.
My ethernet-connected test shows 163.63Mbps.
My wifi-connected test shows 65.62Mbps (using 2.4GHz connection, 40 feet between laptop and router, 6 other wifi devices connected to the same router, all using 2.4 GHz).
Attachment 193347
I consider these results fairly typical.
By comparison, you're paying for a 100Mbps service.
Your ethernet-connected test shows 123.01Mbps.
Your wifi-connected test shows 48.56Mbps but you say it's normally 15Mbps to 25Mbps.
IMO there's 2 issues:
1. Why is your wifi connection speed relatively poor when your wifi-connected device is only 15 feet away?
2. Why the wide variance between 48.56Mbps and 15Mbps?
Issue 1: Maximizing connection speed
Interference - Are there any other devices using 2.4GHz? (Cordless phones, baby monitors, VOIP/wifi phones, Bluetooth devices, nearby car alarms, satellite TV decoders/video senders, etc? Are there any nearby neighbours using the same wifi channel?)
Obstruction - Is this clear line of sight or are there any walls in between.
Absorption - Fish tanks, people, radiators (basically, anything with water in them)
Swamping - How many other wireless devices are in use (laptops, cell phones, tablets, etc.)
Wifi device drivers
Issue 2: Consistency/less variance
Similar to the mainly static conditions above but more variable, i.e. people/animals moving about.
Hope this helps...