Hey @danilvoe, an excellent question!
The channel member list loads up to 100 users and displays "..." at the bottom of the list to indicate that there are more users on the channel. This was done to avoid performance issues downloading 100s of users, as you referred to.
We have a Help Wanted ticket in GitHub open for a community contribution to direct users to the "View Members" modal after clicking "..." on the channel member list. This way they can easily browse through all channel members if they'd like (the modal is "Manage Members" for admins).