Mac Pro (Early 2008) wifi running really slow compared to phone in same room on same network

Hold down the Option key while you click on the Wi-Fi Icon in the menuBar. This will give you a snapshot of your current conditions:

Within a few feet of your Router, with clear line of sight, you should have better than (closer to zero than ) -50 for RSSI or signal.

Other numbers may indicate disconnected or mis-connected antennas.

The Mac Pro 2008 is an ancient computer, it therefore has a correspondingly old WiFi card as standard. That speed therefore might genuinely be all that your old WiFi card can achieve.

Note: No network connection has 100% efficiency not even a wired Ethernet one so even if say your WiFi card is capable of a maximum of 54Mbps which is the maximum 802.11g supports you would never get 54Mbps throughput. (I believe the Mac Pro 2008 originally came with a 802.11n capable card.)

It is possible to replace the original WiFi and Bluetooth cards in the Mac Pro 2008 with newer models which would support higher speeds, I did this myself with my Mac Pro 2010 model and upgraded to an 802.11ac capable card.

See this for an upgrade kit - https://www.osxwifi.com/product/mac-pro-2008-31-2007-21-2006-11-apple-broadcom-b cm94360cd-802-11-a-b-g-n-ac-with-bluetoo…