Of course plenty of bugs remain, no software written by man is bug free
But those kinds of bugs are not "deal breakers" the software functions, those are what we could consider visual bugs. A larger issue with the program is its use of GetTickCount() a 32-bit Windows timer that after 49.7 days, resets to 0 and causes WinMX to completely flip out and become unusable until restarted.
Anyway as for the speed issue, I run a few 1Gb/s servers on WinMX and the issues are mainly down to disk access issues. The caching size for file chunks is not large enough causing disk thrashing and WinMX relies too heavily on its dat files which seemingly are read and written to constantly while the program is open.
If these dat files are on the same disk that is being accessed for file transfers it causes all kinds of glitches due to the slow access of the dat files caused by the overloaded disk. I've found the program runs much better with high speed networks if you use an SSD as a result.