The right place to ask questions about tn5250 is on the tn5250 mailing list. All of the developers are on that list as well as many other helpful users. Get on the list by visiting http://www.midrange.com
and selecting the linux5250 list.
As for the slowdowns, I can pretty much guarantee it isn't tn5250. I should know - I'm one of the developers!
You may find the tn5250 HOWTO useful as well. Grab it from http://www.chowhouse.com/~james/tn5250-HOWTO.pdf
There are some example keymaps in there. IF you have any questions about the HOWTO, just ask me since I wrote it.
Finally, you may like x5250 over plain vanilla tn5250. Depending on whether or not you enable the enhanced 5250 protocol x5250 can be quite different from tn5250. x5250 uses the same backend as tn5250 so they have the same feature set. x5250 does have some nice things that tn5250 doesn't. Of course I may be somewhat biased in favor of x5250 since I wrote that too. Check it out at http://www.chowhouse.com/~james/x5250
p.s. - please do get on the mailing list and let us know how your project works out for. Many of us are interested in hearing about how linux and tn5250 have been implemented in various places around the world. If it works out for you it would be nice to know about it as others can use your experience to guide their own installations.