2015 MBP's have 802.11ac WiFi. And it cannot be upgraded anyway.
WiFi is almost always slower than wired ethernet. And it is affected by many things including signal strength & channel congestion. Signal strength depends on router capability, atmospheric conditions and the distance & any walls/objects the signal must pass through. Congestion depends upon how many nearby networks use the same channel and how many users are connected to them. It is not uncommon for WiFI to operate at less than half of its "rated" speed and to vary over time.
You can check for channel congestion. Option-click the WiFi icon in the menu strip, then select Open Wireless Diagnostics. (But DO NOT actually run the Diagnostics). In the Wireless Diagnostics menu strip, click Window > Scan, then enlarge the window and click the Channel column to sort by channel. If your MBP is using the same channel as other networks there is congestion; and the more networks using the same channel the more congestion there is.
Most routers auto-select the clearest channel depending on what they sense in the surrounding radio environment; but it doesn't always work to best advantage. I have found it best to do a network scan and then manually select an unused channel (configured in your router, not your Mac).
On 2.4GHZ the ideal (non-overlapping) channels are 1, 6 & 11 and you should use the least congested one of them. On 5GHZ there are no overlapping channels; most people use 36, 40, 44 or 48; however I have found channel 161 to work most reliably at the fastest speed. Ideally what you want is a channel no one else is using; you set the network channel in your router (not your MBP).
