@Wowfunhappy Are you using transmission 2.94 (which is the last official binary release to support 10.9). I'm seeing increasing issues where transmission is simply unable to discover as many peers as libtorrent based clients (e.g. deluge, qtorrent).
After some searching, I believe this is because transmission (even the latest version) doesn't support the BEP hole punching extension, which libtorrent-based clients and utorrent support. This is particularly bad for poorly seeded torrents since it means that torrents will never be able to make progress. And the increasing prevalence of VPN and people who don't know how to set up port forwarding means that this is likely having significant impact.
In terms of alternatives (not even limiting to things compatible with 10.9), the ecosystem as a whole isn't very good. qbittorrent and deluge exist (and both use libtorrent internally so should be identical performance-wise) but qbittorrent is a bit of a mess code-wise (comparatively transmission has a clean design and is easy to read). And deluge is a python gtk app with a very non-native gui. The cli-based rTorrent also doesn't support BEP-hole punching and is explicitly designed more for seeding rather than DL purposes.
While µTorrent was once a very lean and native app [on both windows and mac], µTorrent was sold to an adware company and ironically became the very thing it was created to spite. (And even the last "classic" version for mac, 1.8.7 still came with advertisement and popups by default. Which is a shame really because it does have a very nice native gui. While one could patch out the badware [and such copies are floating around], I wouldn't really be comfortable running it because there's no way to know how deep the badware truly runs (most copies you find online only remove ads. In my own disassembly I found the application has some codepaths to show surveys, popup a sponsored browser, send IP addresses to some random hardcoded endpoint hosted on ec2, and that's only just the obj-c part of the app!).
For 10.9, the options seem to be: Transmission 2.94 (which suffers from the issue noted), Transmission 3.0 (this should technically compile from source fine, since there were no breaking changes on the mac part from what I saw, but isn't really any better connectivity wise), Deluge 1.3.15 (works fine but is really ugly), and qBitTorrent 4.1.3 (it's sort of ok as a qt app, but nowhere as nice-looking as transmission).
Since QT5.9 can apparently work with mavericks it might be possible to get up to qBitTorrent 4.4 working (which importantly has support for BTv2 torrents which might be good for future-proofing).
@bbbc For BiglyBT it seems to be java based. It's possible to get up to JDK11 working on mavericks fine, with GUI. Supposedly even higher versions can work, I think one of the macrumors threads about minecraft showed a way to get JDK17. What concretely is the issue in getting it to run, have you tried just swapping out the provided jre for your own?
Edit: Actually it may not be related to BEP55. I just tried rtorrent which not only doesn't implement BEP55 but doesn't implement uTP protocol entirely, and even that was able to find more peers. On a whim I saw that Transmission does have an option to disable uTP, and with it disabled Transmission was able to find the same peers as rTorrent. So there's something wonky with Transmission's uTP implementation. I didn't actually realize uTP was a thing, I thought all BT traffic was udp by default, I'll have to read more about this...
www.biglybt.com
