@Dpikau 

Hi,

Have you added any new electrical appliances or electronic devices to your home around the time when the speed dropped to 10 Mbps? (may be anything that is permanently connected to the electric circuit of your home, no matter of its size)
 

  @woozle Hi, thanks for replying. No, no new appliances have been installed at home.

  @Dpikau 

Troubleshooting this kind of problem is cumbersome.

If possible you could try the following.

First plug both TL-PA7017P into two wall sockets that are right adjacent to each other and test the resulting speed. (a portable laptop computer would be of advantage here)
In this condition roughly 300 Mbps should be possible with "AV1000" grade Powerline adapters. If the speed was much lower, then one of the TL-PA7017P might be faulty.

If the above test turned out good, then move the one TL-PA7017P back to your bedroom and unplug everything that is plugged into any of your home's wall sockets, bar the Internet router and the two TL-PA7017P (and the computer, if you don't own a battery powered one).

If the speed now went back up to 100 Mbps, then one of the unplugged devices might suddenly have started causing more interference than before.
If the speed stayed at 10 Mbps, even with all your home's devices unplugged, then there isn't really anything you could do about it.
 

By the way, I also remember other users reporting that when their electricity supplier replaced an older power meter with one of those new "smart" power meters it had a huge impact on their Powerline networks. I guess you would know if such a replacement had happened at your home.