What is your client?
I have used the uPnP / DLNA server in MC since maybe MC11 and have not noticed any major delays in browsing the server.
I use a Denon 4806Ci for Audio only, a WD TV Live Plus, LG ST 600, Media Connect on the iPhone / iPad (Plug Player when it wasn't crashing) and have used XMBC and Plex as clients in the past as I've tested various options.
I have Twonky as part of a NAS I use to backup my server, but Twonky is disabled except when i played a bit with it. Did not notice the difference in speed when using the clients. MC is so much more customizable from a menu perspective, although Twonky does seem to support a huge array of clients, the biggest benefit with MC is how it manages my library for tagging etc. Cannot live without it.
I would suspect maybe a DLNA setting is incorrect on the MC server configuration that maybe upsetting your client, but that is just a wild guess.