it seems the problem is present from version 5.2.5 and above, however I noticed during my tests that it only occured with CoreFTP server for me, I tested it with Solarwind FTP server and everything worked smoothly, actually the newer versions were faster with the latter. then I tested with Psftp 0.64 client and CoreFTP but it didn't had the problem. I doubled checked for CPU-bound bug and visually my CPU usage is very low (on all cores).
I included 3 logs
1- 5.2.5 with CoreFTP (has the problem)
2- 5.2.4 with CoreFTP (OK)
3- 5.2.5 with Solarwind Server (OK)