I had a similar problem. After some time the connection speed from the MacBook to the router (WiFi) dropped dramatically. Using the 'ping' utility on the command line it showed a lot of dropped packets. Restarting the MacBook only did the trick for some hours/days.
I found out that the 3rd-party program 'Android File Transfer' was the culprit. After deleting it, everythin is fine, again
If you do not use this program, try restarting your Mac. Else, try restarting the router. If this fails, too, then try out an ethernet connector and see if the problem is still persisting.
Also, it doesn't hurt to check if the drop in speed is also present when using resources on your local network (try browsing the web page of your router, for example). If so, it is definitely your WiFi, and not your outbound connection.