This is often a server-side issue with the IPTV provider. Try switching from Wi-Fi to mobile data to see if your ISP is throttling the connection.
VLC for Android works fine for basic IPTV, but don't expect a "cable TV" experience. For daily IPTV with many channels, use a dedicated IPTV player. vlc android iptv
VLC is a perfectly legal tool. However, the legality of IPTV depends entirely on the source of the content. Using VLC to watch free, public broadcasts or services you pay for is legal. Using it to access "pirated" premium channels via "gray market" M3U lists may violate local laws and terms of service. Always use a VPN if you are concerned about privacy while streaming. This is often a server-side issue with the IPTV provider