According to your post the difficulty is with your wireless system and not the direct wired connection. As you know, your router really has two separate components and it. One is the switching circuitry used to manage your local area network over Ethernet. The other part is a wireless transceiver that transmits either on the low or high band of the 802.11 standards.
Normal speed for a wireless connection is usually 54 Mb per second. If it drops down to the speed below 10 Mb per second, it would indicate there is something wrong with the wireless connection. One possibility is interference from other devices in your neighborhood. The other possibility is the transceiver inside your router has gone bad. Another possibility is a broken antenna, but I'm sure you would've noticed that.
Here are some general observations about wireless. There is no guarantee that wireless connections will always be reliable. In fact, that is implicit with unlicensed devices. With all unlicensed devices there is the risk of unwanted interference. This is because the devices are not registered and nobody has a claim to a certain frequency. From time to time some individuals either deliberately or unintentionally installed defective wireless devices which cause havoc to other nearby wireless users. Normally the power level of these devices is so low that it does not cause any serious interference.
There have been some notorious cases where individuals making their own experiments have caused serious i nterference to whether radar systems at a certain airport. Of course, that was rare, but it underscore the fact that unlicensed devices can and do cause serious interference. The case in question was because the experimenter was using high gain antenna to extend his range beyond the normal 100 m limit.
But in your case, the first thing I would suspect would be your router. You could try using a spare router from one of your friends. It doesn't have to be a real sophisticated router. Most all of them have the same power level and same capabilities
In fact, you can buy some of the older routers at bargain prices. Sometimes less than $20 for one of the older routers that only works on the low been. Which is okay, because most people like the unit lower bed anyhow for its longer-range through walls and foliage.
Also, in some cases, one of your laptops or computers and heavy defective wireless card that is causing interference to everybody else. You could try a process of elimination by using just one computer at a time and see which one introduces errors into the system.
Is this of any help?