I had the same issue. With Comcast XFinity 25/5 plan, I was getting 16/5. Essentially it was 8 or 9 MB/s less than actual speeds I get while connected using ethernet. After lot of reading, found out that WMM is the issue in the Linksys Cisco EA6200 wifi router.
Went into "Media Prioritization", turned it ON, then went into Settings and unchecked WMM and the speeds came up to 25/5 as it should. It is important to note that even though Media Prioritization is OFF in its default state or when you configure router from scratch, it still somehow applies WMM unless you manually go in and turn it off.
I would expect a company such as Linksys Cisco to make routers that don't throttle down in this day and age but unfortunately, Linksys Cisco does not do things right!