The only spec that Apple ever posts about WiFi is that their devices are WiFi 802.11 a, b, g, n, ac compatible.
No actual, definitive Wifi data transfer speeds.
The iPad Mini 3 spec is listed as compatible with the WiFi 802.11a/b/g/n; dual channel (2.4 GHz and 5 GHz) and MIMO standards.
Wifi 802.11 ac speed is spec'd at data transfer speeds of, at least, 300-400mbps and up to a couple of Gbps.
The iPad Mini 3 is only spec'd at Wifi 802.11 n with a max data transfef speed, which is only “up” to 300 mbps.
So, that speed may vary in your 5-year old iPad Mini 3.
Sometimes that iPad Mini 3 may reach the max 802.11n data speed. Sometimes it won't.
Plus that iPad Mini 3 is your oldest iDevice, as it is nearly 5 years old, now.
So, it is probably going to be the slowest WiFi device in your lot.
Also, your other wireless devices, as well as other Internet “traffic” maybe cutting into your WiFi/Internet bandwidth, as well.
Best of Luck to You!