Just those connected to the repeater... but for many users it doesn't really matter.
First off the effect is only related to your local Wi-Fi speed. I for example have a 35Mbps Internet connection to a Time Capsule, and an AirPort Express set up as wireless repeater. Machines on the wireless repeater still get about the same Internet speed as ones plugged directly into the Time Capsule with gigabit Ethernet. For my iOS devices doing their "Stuff" - backing up to iCloud, etc. - no difference at all.
Where I do see an effect is in local traffic, e.g. the backup speed from a Wi-Fi connected Mac to the Time Capsule. If that machine were connected direct to the Time Capsule without an extender, the backups would run faster - but they run fast enough, such that I don't notice any difference in performance in the real world.
My Apple TVs are all close to the TC itself so probably not running through the extender - if they were, I suppose it's possible that HD streaming from my iTunes library could be less stable, but I doubt even that would really be a visible effect.