Fast WiFi immediately after router reboot, slow WiFi some time after it

Question

I have an OpenWrt D-Link DIR-825 router operating at both 2.4GHz and 5GHz. There are four wireless devices: a HP OfficeJet 8620 printer, NVIDIA Shield Tablet, Samsung Galaxy S5 Neo and my laptop, HP ProBook 430 G4.

I'm used to the low 2.4GHz Wi-Fi performance. However, when I reboot the router, and access the network immediately after reboot using my HP ProBook 430 G4 by visiting some website in the same country (thus having low latency), the website loads very quickly, perhaps 2-3x faster than usually.

However, when I wait for a while (a few minutes or so), the website load latency will be back to its usual levels (with a large website taking 5 seconds to load). I assume that somehow the additional devices (printer, tablet, phone) that attach to the wireless network slow down the performance of the network.

What might be the cause of the other devices slowing down performance? Is it possible that some of the devices doesn't support the newest Wi-Fi standards and thus every device including my ProBook is forced to use an older Wi-Fi standard?

Pinging the OpenWrt device shows rtt min/avg/max/mdev = 2.377/4.706/6.933/1.205 ms in the slow mode. I don't know if this latency is better in the fast mode, but I can certainly run some additional test if that helps solving the problem. But the ping time in my opinion shouldn't explain the slowness, as the latency is so low.

Downloading a large Linux ISO image in the slow mode is done at performance of 9.6 MB/s, or 76.8 Mbit/s whereas the Internet connection speed is 100 Mbit/s (I haven't tested this in the fast mode, but can certainly run some additional tests). So at least large file download performance doesn't explain the increased response time.

I found at least this similar question:

• Router slows down after some time, only over wifi, no admin, router reboot helps

...but the answer was to try OpenWrt, which is something I have already done.

Edit: I tested 5GHz. It definitely feels much faster even though I haven't rebooted my router recently. The results are rtt min/avg/max/mdev = 3.436/4.240/4.607/0.453 ms, so perhaps the cause isn't latency but rather jitter?