With Version 5 of Virtual Earth unoffically released as part of the upgrade of maps.live.com I have had a quick look at the performance and it is great news! Compression has been enabled for not only V5 but also V4 seeing that download 1/4 in size! Version 4 just went from 447KB to 106KB while V5 at 581KB will never be an issue as it is compressed to 139KB.
But that is just the start of it, by disabling the download of the ring of tiles not visible to users we see less images tiles downloaded on startup, less KB and more importantly the visible tiles loading first.
Have a look at this comparsion on a simple 400x400px map on Aerial mode with some detailed tiles:
First up is V4:
Now V5:
Notice the difference? 9 tiles compared to 25! and a saving of 430KB!
At this point the keen eye will pick up on something else, where are all the little images for the dashboard? Well check this good idea out:
I haven't heard anything about this but it appears that the smart people at Virtual Earth are using a single image, to be cached by your browser, and used in multiple places.
The final scores are:
V4 pre Today: 1155KB (est)
V4: 814KB
V5: 415KB
As a VE developer I press crtl F5 (gets all new files) about 50 times a day (or more if I am working in CSS pixel pushing hell) So i could personally see a saving of 1GB over a month!