Depende realmente da distribuição do linux que você usa como modelo do Docker, mas isso não explica os 100%.
O Systemd-logind não deve usar tanto CPU como o trabalho:
https://www.freedesktop.org/software/systemd/man/systemd-logind.service.html
systemd-logind is a system service that manages user logins. It is responsible for:
Keeping track of users and sessions, their processes and their idle state. This is implemented by allocating a systemd slice unit for each user below user.slice, and a scope unit below it for each concurrent session of a user. Also, a per-user service manager is started as system service instance of [email protected] for each user logged in.
Generating and managing session IDs. If auditing is available and an audit session ID is set for a session already, the session ID is initialized from it. Otherwise, an independent session counter is used.
Providing PolicyKit-based access for users to operations such as system shutdown or sleep
Implementing a shutdown/sleep inhibition logic for applications
Handling of power/sleep hardware keys
Multi-seat management
Session switch management
Device access management for users
Automatic spawning of text logins (gettys) on virtual console activation and user runtime directory management
Suponho que há algo errado com seu modelo de janela de encaixe.