@loftar: Firstly, thankyou for the detailed reply and explanations.

To clarify, I am also experiencing spiking network lag during gameplay. I'd categorise this as: movement "rubberbanding", delays in click/action responses (etc), which occur frequently
but intermittantly. For example, I can be experiencing smooth <1s response times for a few minutes, and then suddenly be waiting 10-20 seconds (or even longer!) while my character walks in place and nothing responds. These spikes usually occur every minute or two. They were endlessly frustrating at first, but I've learnt to tolerate them... mostly. xD
Anyway, as for the topic, if I'm understanding this right... the sprites are downloaded, but then cached and stored locally for future use. But the problem is that they're being loaded unusually slowly, not that they're being
re-downloaded each time?
I'll also check to see whether there's any java updates I can grab, as I have no idea what version I have installed. I'll report back if that helps.