Realtek 8821ae wireless very slow

Ok, at least that out of the way. Just to be clear, you assert other laptops connecting to your router via wifi DO NOT show this symptom...
While I cannot be a 100% sure of this, since the issue happens sporadically, my friend uses his laptop a lot on this network and haven't experienced this issue.
 
Here's my theory:
  1. the ISP connection should be within the ISP domain range 83.0.0.0 - 83.255.255.255
  2. it can move, but only within that range
  3. your performance is reasonable when it is and miserable when not

when things are performing nicely (both wired & wifi), run TRACERT -d 216.58.211.132
  • I contend that the #2 node will be within 83.0.0.0 - 83.255.255.255
when things degrade run it again TRACERT -d 216.58.211.132
  • I think #2 node will not be in the range 83.0.0.0 - 83.255.255.255
 
Here's my theory:
  1. the ISP connection should be within the ISP domain range 83.0.0.0 - 83.255.255.255
  2. it can move, but only within that range
  3. your performance is reasonable when it is and miserable when not

when things are performing nicely (both wired & wifi), run TRACERT -d 216.58.211.132
  • I contend that the #2 node will be within 83.0.0.0 - 83.255.255.255
when things degrade run it again TRACERT -d 216.58.211.132
  • I think #2 node will not be in the range 83.0.0.0 - 83.255.255.255
My wireless connection only degenerates when I don't want it to, so that'll have to wait until I find an opportunity. I can however deny your first prediction. When things are performing nicely the #2 node is 100.70.64.1 for both wired and wireless. I suspect this won't change when things degrade.

Wired:
Tracing route to 216.58.211.132 over a maximum of 30 hops

1 <1 ms <1 ms 1 ms 192.168.1.1
2 1 ms 1 ms 1 ms 100.70.64.1
3 2 ms 1 ms 2 ms 10.33.247.209
4 1 ms 1 ms 1 ms 100.64.3.4
5 8 ms 9 ms 9 ms 89.160.55.222
6 9 ms * 9 ms 100.64.11.1
7 9 ms 9 ms 10 ms 83.233.255.78
8 10 ms 10 ms 9 ms 216.239.43.122
9 9 ms 10 ms 9 ms 108.170.235.63
10 10 ms 10 ms 9 ms 216.58.211.132

Trace complete.


Wireless:
Tracing route to 216.58.211.132 over a maximum of 30 hops

1 3 ms 1 ms 1 ms 192.168.1.1
2 5 ms 3 ms 2 ms 100.70.64.1
3 4 ms 5 ms 5 ms 10.33.247.209
4 12 ms 11 ms 2 ms 100.64.3.4
5 10 ms 9 ms 17 ms 89.160.55.222
6 11 ms * 68 ms 100.64.11.1
7 12 ms 11 ms 12 ms 83.233.255.78
8 12 ms 21 ms 106 ms 216.239.43.122
9 11 ms 11 ms 12 ms 108.170.235.63
10 10 ms 10 ms 11 ms 216.58.211.132

Trace complete.
 
Good to see at least the gateway is consistent!

If you could get an identical pair when bandwidth drops that would be of assistance.
Meanwhile here's a side-by-side look at the two interfaces
upload_2017-5-26_7-48-8.jpeg
The routing paths are the same but timings vary interestingly.
node#1 is from your PC to your router and it should be very low as shown by the Wired connection.
The WiFi takes 3ms to arrive at the router.

Other significant variations are in red
 
It's a laptop. Can't exactly pry it open and replace stuff. I have tested on my desktop, my friends laptop and my phone, none of which have this issue.
That's not what I meant; have you tried using a wireless adapter with an external antenna, such as one that connects via USB?

Obviously other devices will not exhibit the issue if it's the specific antenna on that laptop that's defective.

Good to see at least the gateway is consistent!

If you could get an identical pair when bandwidth drops that would be of assistance.
Meanwhile here's a side-by-side look at the two interfaces
View attachment 82375
The routing paths are the same but timings vary interestingly.
node#1 is from your PC to your router and it should be very low as shown by the Wired connection.
The WiFi takes 3ms to arrive at the router.

Other significant variations are in red
Those results point to some sort of wireless signal attenuation taking place.
 
Those results point to some sort of wireless signal attenuation taking place.
I was thinking the wifi was slow even getting to the local router, so I tested my own and found very similar results on a well behaving Dell Laptop - - so it's elsewhere.
 
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