Ok well to be honest this sounds more like throttling but you have to bear one thing in mind about this, most ISPs in most parts of the world do some kind of throttling, so even if its not Your
ISP doing it it can still affect you when trying to get files from those users.
One thing that strikes me about your post,
under Exceptions- I have Winmx1 tcp=16,699 and Winmx1 udp=16,257
The obvious thing to mention here is 'DONT' use any commas when you input the actual numbers into firewalls or routers, but you also said this:
In Winmx, I have the following settings: TCP=5051; UDP= 10,000.
This will never work and if its actually how you have it set up I cant see how any of your uploads would ever start.
So you've posted in the WinMX Friendly ports area let me give you some advise for ports to use
WinMX TCP = 52300
WinMX UDP = 53200
The trick is to be absolutely sure that you have the same numbers used in both your firewall, the router rules, and in WinMX > Settings > Incoming TCP and In/Out UDP.
The other thing I'd like for you to do while your adjusting things is to test your speeds with a script on site here, and follow its recommendations as to how to set an Outgoing Limit in WinMX.https://www.winmxworld.com/scripts/bandwidth/
This needs to be done with WinMX off and all web pages and other bandwidth using sites or programs closed, then reopen WinMX when its finished testing to adjust it as per the instructions. Limiting your uploads to a certain speed can insure that not all of your speeds/bandwidth is taken that way and enough is allocated to your downloads.
Next I'd like you to make sure that your Auto Enter Que and Auto Find Sources limits are set properly.https://www.winmxworld.com/help/winmx-settings-search.html
Often users with larger files have longer queues and this can cause you problems as WinMX is only set by default to enter into ones as short as 10 or less. Setting this higher may mean you have to wait but at least your in line.
Again keep in mind here that files that start great and then peter down to 0.00 and then die this is a tale tell sign of ISP throttling, because your uploads dont do this, it leads me to believe that some of the users your finding for sources do infact have this problem, often will be left with a message 'Connection Reset By Peer' when infact its the ISP interferrance that is causing it and its no fault of yours or thiers, something to keep in mind.
Last you may want to go to a room and test downloads with someone that you are 'sure' has no limits or throttles and see what kinds of speeds you can get. If you need a room to do this in there are usually folks around in the
There you can go in and tell them you need to test download speeds on MX and if anyone is around that can help we will.
Let us know how you get along with all this...