First, in FlashFxp try connecting via 127.0.0.1 and check the "use site IP for PASV" option to see if that makes a difference. My guess is this should solve the problem because as I mentioned using the external IP and having the router/modem forward it can sometimes be slow.
If that doesn't fix it, make sure you're not using SSL and see what happens. Then try installing the free version of serv-u or whatever and see if that also has a performance problem locally. It's possible you've got some really slow anti-virus, etc so check the CPU meter to see if anything really busy on any of the cores.
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