Streaming delay variation reasons?

I stream to BOSE Soundtouch an have about 6 seconds delay, which is HUGE compared to other posts here and I am trying to figure out why.
My PC is connect by 1GB Ethernet, so no issues to expect here. The WiFi is directly connected to the same hub and the SoundTouch 10 is standing directly beside it.
I DO acknowledge having troubles with connectivity of the soundtouch devices with my WiFi, (which is why I was looking into Sonos products, discovering SWYH in the process) but I never got hold of anything.
Switching MP3 to PCM didnt change anything, so maybe SWYH can help me debug this anyhow? Any Log I can turn on, any (hidden) settings to tweak?

Thx in advance

PS: Sonos will get a shot next month :wink:
PPS: Having 5 rooms makes this quite an expensive project so I am very carefull with this change