The following is not a criticism of PD, just offering some speed test results. I have no agenda. YMMV, these are the results I got. I ran some download speed tests of various products, Windows and Mac. The download was the 10.9.5 combo update from Apple. I chose this file because it's large (almost 1 GB) and hosted on Apple's servers, hopefully eliminating slowdown due to host server. I ran the tests around between 1am and 2am to eliminate overall web traffic as a factor and slowdown on my ISP's network. I have a rated 50Mbit line, so that should not be a factor, and prior to starting the tests, I ran Speedtest.net and got 49 Mbs down and 7Mbs up, which is nominal for my ISP connection outside of peak times. For each test scenario, I ran it three times, the times shown are the lowest of the three tests (fyi, there was minimal variance in the three tests for each setup, so IMO the times shown accurately reflect the efficiency and impact of the download manager, and minimal influence due to Apple's Server, internet traffic in general, or my ISP.
Progressive Download-Mac 3:55 (3m 55 sec)
Firefox Mac using FF built in download manger 3:20
Firefox Mac using DownThemAll FF plugin 3:00
Free Download Manager 3:22
FF Win using FF built in download manager 3:15
FF Win using DownThemAll FF plugin 2:55
I used PD with all default settings OOTB, freshly installed, same with DownThemAll. From what I could see, PD was establishing multiple connections. I had first tried PD back in the Snow Leopard days, had pretty much the same conclusion, that there were faster alternatives. I just happened to notice today on MAcUpdate that PD had been updated and thought I'd try it again. If you are Firefox users, you should check out DownThemAll (unavailable for Safari or Chrome). I will say that PD definitely shines in many areas (restart, resume, scheduling, filtering) so by all means there is more to consider here than just speed.