That's not how instancing works.
The load on the back end is based on total playerbase numbers alone -- how those numbers are distributed across platforms has no bearing on performance. A web request is a web request no matter what type of device it came from.
The real time sessions are based on p2p hosting so one player acts as a server for
1..n players,
n being some max number of players per session set by the developers (edit to add: or
n is determined algorithmically at runtime based on measurements on how the host and the clients are coping with their current load). There are
1..m such sessions for each game location that's currently occupied by players. These are called instances.
Instances do not directly cross-communicate any real-time data that needs to be updated frequently like position or orientation, so the total number of instances doesn't impact performance either. Player actions may modify persistent data stored in the back end and those effects propagate across instances that way, but that rate of updates is small enough to be negligible.
The number of instances per game location depends on instance occupancy and matchmaking rules such as Open/PG/Solo, blocking,
platform, etc.
Poor performance when there are other CMDRs around are an indication of some host or client having performance issues in general. It
is an orthogonal problem to matchmaking rules which is what crossplay essentially is.
I have designed, developed and am now managing the development of a rather similar system and we match Oculus Quest users with MacOS and high end Windows PC users on a daily basis. I just got my first US patent on multiplayer networking technology in 2020 so even if I may not be exactly on point for 100% of the implementation details, I know a little bit about multiplayer networking and none of ED's networking tech appears to be exceptional in any way.
Therefore I argue that stupid implementations aside, crossplay has no
inherent performance implications.
I didn't say they are going to be sorted out, I said they
should be sorted out. As they obviously should.