Yep, that was it for me too. I adjusted from the default 8 to 24 and now: (10:33:00) [i] transferred 8 file(s) - Total: 26.4 M byte(s) in 33.14 (836.56 KBps) Glad we got this resolved. I really didn't like the other client I was using. :)
I was having the same problem on a Win7 system, i.e. my upload speeds were only 1/3 as fast as with Filezilla. I changed my tcp/ip buffers to 1024 each and now the speeds are the same as with Filezilla (35 Mbps+) with all the advantages of FTP Rush.