Personal experience for me; my fire stick buffers all the time. Sling TV, IPTV, Prime Video, with or without VPN. I think others have had this issue too when using the firestick.
I dont have an issues with fire t.v. products. I use a netgear night hawk for my Wi-Fi and can watch my service flawlessly anywhere in my home.
The hardware you have is very fast and capable of handling playback of FHD and 4K streaming.
This is an issue with your provider, do a ping test to their servers to check for latency. Most likely your provider server is overloaded.
I use tivimate and firestick 4k. Internet speed at 65 Mbps up and down with 5ghz router. My firestick is also rooted and I disabled most of the Amazon bloatware. I find the performance comparable to my nvidia shield when watching IPTV. My service provider does buffer sometimes too because I when I load it on Nvidia shield it will buffer too. So it's provider issue. My firestick4k video playback settings is set with a LARGE buffer and SOFTWARE video decoder.
Use a vpn it helps
Change your dns to 9.9.9.11 or 1.1.1.1
I had a lot of buffering on my Fire Stick 4K until I added a wired connection and ditched wi-fi. It runs much better now. I also use Tivimate.
Try using my free iptv on the first channel (no buffer)
try loading your iptv through VLC on your computer ..
use a wired connection from modem to laptop and disable wifi.
if it still buffers alot then it's either your isp is throttling your speed or the servers for the iptv are too far from you causing high ping
Firestix are prone to over heat which can cause buffering as well from what I have read.
I use a cheap $30 android box and never have any problems .. I use IPTV smarters app free on playstore