Hi SimpleSim,
I've got an original Model B Pi running OpenELEC 3.0.0 (i.e. a build of XBMC 12.0), playing files off the Hummy's default DLNA server and also off my router's in-built DLNA server.
Startup performance: About 20 seconds from power-up to being responsive. That includes the XBMC boot waiting for the wireless to be powered up and get connected.
Playing performance: Perfect HD and SD. No glitches, buffering or drop-outs while playing, even on high-bitrate BBC1 HD. HD takes about 10 seconds to buffer before playing; SD about 3. Can't FF or RW though - doesn't really bother me as most of our recordings are BBC.
Software config: Just the basic OpenELEC 3.0.0 RPi build. Added the SD license codec, bought from
www.raspberrypi.org. Some non-essential tweaks I did included changing the CPU/GPU memory split in the Pi bootloader config (can't remember exactly but I can look it up if you're interested).
Control method: Pi is running without keyboard/mouse; TV too old for CEC control via remote; so using Android phone with UPnPlay (free) to queue the videos up. This is great because I can use the phone to browse videos rather than the XBMC front end - it just seems easier to me. If I need to control XBMC in finer detail, e.g. for config settings, then I use Yatse on Android as a remote control. There's no lag on the controller at all.
Network config: Pi is on ethernet cable into an 802.11n access point, which is running in "client" mode. That has a directional antenna pointed at my wireless router, which then relays to another 802.11n AP in client mode, which is then ethernet cabled into the Hummy. The directional high-gain antennae are absolutely crucial - performance was abysmal without them, albeit over about 12 metres through several walls a a floor...
Please excuse the crudity of this diagram...
Power config: Pi and client AP are automatically powered up when the TV turns on - regardless of whether it's needed - by being in one of those smart extensions leads. Likewise when the TV goes off, the Pi's power just gets cut abruptly... Seems to work OK, no corrupted file system so far (!)
Hope that helps.