Fluctuating WiFi latency

Question

I am trying to troubleshoot my unreliable WiFi, but I am lost. My wired connections work flawlessly, but my WiFi has massively fluctuating latencies. This occasionally knocks out my Chromecast.

Here is a sample of ping output to illustrate the observed behavior:

64 bytes from 192.168.1.101: icmp_seq=37 ttl=64 time=100.089 ms
64 bytes from 192.168.1.101: icmp_seq=38 ttl=64 time=1.107 ms
64 bytes from 192.168.1.101: icmp_seq=39 ttl=64 time=28.129 ms
64 bytes from 192.168.1.101: icmp_seq=40 ttl=64 time=44.433 ms
64 bytes from 192.168.1.101: icmp_seq=41 ttl=64 time=1.230 ms
64 bytes from 192.168.1.101: icmp_seq=42 ttl=64 time=3.609 ms
64 bytes from 192.168.1.101: icmp_seq=43 ttl=64 time=0.912 ms
64 bytes from 192.168.1.101: icmp_seq=44 ttl=64 time=3.684 ms
64 bytes from 192.168.1.101: icmp_seq=45 ttl=64 time=3.690 ms
64 bytes from 192.168.1.101: icmp_seq=46 ttl=64 time=41.451 ms
64 bytes from 192.168.1.101: icmp_seq=47 ttl=64 time=122.275 ms
64 bytes from 192.168.1.101: icmp_seq=48 ttl=64 time=86.806 ms
64 bytes from 192.168.1.101: icmp_seq=49 ttl=64 time=41.053 ms
64 bytes from 192.168.1.101: icmp_seq=50 ttl=64 time=121.413 ms
64 bytes from 192.168.1.101: icmp_seq=51 ttl=64 time=83.308 ms
64 bytes from 192.168.1.101: icmp_seq=52 ttl=64 time=1.076 ms
64 bytes from 192.168.1.101: icmp_seq=53 ttl=64 time=3.748 ms
64 bytes from 192.168.1.101: icmp_seq=54 ttl=64 time=3.661 ms
64 bytes from 192.168.1.101: icmp_seq=55 ttl=64 time=3.658 ms
64 bytes from 192.168.1.101: icmp_seq=56 ttl=64 time=3.926 ms

I am using a D-Link DIR 878 HW:A1 (Firmware 1.04) configured for dualband.

I live in a (suburban) house and should therefore only be mildly affected by neighboring WiFis.

To eliminate potential noise from my ISP, I am deliberately pinging a local wired device (a Mac Mini) with my MacBook. To reduce the probability of the issue being confined to my MacBook, I have run similar ping tests from my Acer laptop running Windows 10 and observed the same fluctuations.

Using Wifi Analyzer for Android, I have scanned the 2.4 GHz band and moved my network to an area (channel 12) with no other visible network.

Additionally, I connected to the 5 GHz network and observed similar fluctuating performance on both laptops (MacBook and Acer).

The moment I plug in a wired connection, the issue vanishes.

I have unplugged my microwave and I have placed my laptops next to the WiFi router to see if performance improves. But no.

This router was purchased after I returned a mildly cheaper TP Link router for fluctuating performance.

I have tried using my Huawei P9 as WiFi hotspot next to my laptops and run a ping test against the gateway - also fluctuating performance. On both laptops. In the house and on the street. Both laptops are connected to power, so power saving optimizations should not impact the numbers.

To make matters worse, the fluctuating isn't even fixed. It's alternates. While I have been writing this question, my laptop now medians around 2-3 ms, but occasionally (as in every 15-20 seconds) jumps to 100 ms for one request.

I don't understand this.

I'm not necessarily looking for a fix, but a relatively comprehensible explanation on why this may happen and what can (or can't) be done about it. I would expect a consistent < 5 ms ping time, especially considering that no other devices are using the networks.

Although I am a software engineer by trade, I have no (academic) understanding of how radio signals work, so avoid overly technical explanations if possible.