If it's lower, there's a reason, such as interference or another RF problem. When your speed seems slower, check to see what rate the radio says it's negotiated with the router (and check the router's view of your laptop too). Rebooting it just resets the error counters it's not actually fixing anything. If the error levels drop, the radios will (eventually) negotiate back up to better speeds. If too many packets error out, the radios will back off to lower rates. Over time, radio link quality changes, causing the radios at both ends to adjust Tx/Rx rates. (It would be interesting to see what the router says, as Tx and Rx can differ.) For example, in your screenshot, your WiFi radio has negotiated a fairly decent RF speed (MCS 9, 867Mbps) with the router. WiFi devices will reduce speeds if they detect too many errors at the higher rates.
