by default I believe qbittorrent defines 500 max concurrent connections, but no per torrent restriction, so a single torrent can have by default 500 connections. I think qbittorrent also has bigger cache by default, but can't remember. Mixed.mode 0 makes it prefer TCP instead of uTP, because that is almost always faster. For instance, qbittorrent by default I believe has enabled multi-tracker announcing and mixed-mode zero, which deluge I believe doesn't, but can with ltconfig. If qbittorrent is faster than deluge, then it's settings differing and/or another libtorrent version used as backend with issue on your end with regard to some setting, or just in general possibly. Not really related, but if starting a magnet download, then transmission and rtorrent takes about 1 minutte to start, because needs to bootstrap the DHT and find connections, and is slow, whereas libtorrent-clients does it in 1-2 seconds, with even empty DHT from start - really impressive imho.
However, sometimes even if you have great internet connections when surfing or streaming, you have trouble with sluggish torrent downloads and how to speed up BitTorrent downloads.
Libtorrent is just as fast as anything else out there, and agruably faster as can be tweaked more to whatever circumstances, and even by default is pretty optimized already, atleast not "slow" setup. To download BitTorrent, users simply go to the BitTorrent website at and click the Download button on the homepage.