A quality=90 jpeg exported from GIMP is ~1.4 million bytes and not obviously visually different. (Test process was loading original image into one Firefox tab, and quality=90 jpeg image into another, holding Ctrl+PgDn to flip between them quickly, and looking at the screen with my eyes to see if any obvious differences leapt out.)
quality=20 (~0.32 million bytes) wasn't obviously different either.
quality=10 (~0.21 million bytes) was noticeably different. And, on second glance, the obviously different areas were actually slightly different in quality=20 too.
I didn't do any more tests. So, they could have made the image less than 10% the size, I guess - but, they can probably afford the bandwidth, and the thing needs to end up fully uncompressed at some point anyway just so that it can be displayed on screen. It's not even like 4 MBytes is a lot of memory nowadays.
A quality=90 jpeg exported from GIMP is ~1.4 million bytes and not obviously visually different. (Test process was loading original image into one Firefox tab, and quality=90 jpeg image into another, holding Ctrl+PgDn to flip between them quickly, and looking at the screen with my eyes to see if any obvious differences leapt out.)
quality=20 (~0.32 million bytes) wasn't obviously different either.
quality=10 (~0.21 million bytes) was noticeably different. And, on second glance, the obviously different areas were actually slightly different in quality=20 too.
I didn't do any more tests. So, they could have made the image less than 10% the size, I guess - but, they can probably afford the bandwidth, and the thing needs to end up fully uncompressed at some point anyway just so that it can be displayed on screen. It's not even like 4 MBytes is a lot of memory nowadays.