WiFi devices will reduce speeds if they detect too many errors at the higher rates. For example, in your screenshot, your WiFi radio has negotiated a fairly decent RF speed (MCS 9, 867Mbps) with the router. (It would be interesting to see what the router says, as Tx and Rx can differ.)
Over time, radio link quality changes, causing the radios at both ends to adjust Tx/Rx rates. If too many packets error out, the radios will back off to lower rates. If the error levels drop, the radios will (eventually) negotiate back up to better speeds. Rebooting it just resets the error counters; it's not actually fixing anything.
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). If it's lower, there's a reason, such as interference or another RF problem.
The 802.11ac standard also supports MIMO (multiple in, multiple out), which requires all devices to utilize more than one antenna to get maximum throughput (which is somewhere above 1Gbps I believe). There could be a situation where the AP radio is negotiating just a single channel with your laptop and multiple channels with other devices that come on after it does.
Without knowing what kind of router you have or how many other devices are on the network, it's hard to say what else could be causing the slow-down's/back-off's.
On most routers there are advanced wireless settings that can affect how clients interact with the router. In my experience, it is useful to disable the fancy settings (or setting them to defaults), then test each one until you find the culprit.
