Scalability of concurrent log restore for different databases on SQL Server
For 1 &amp; 2 - You need to take into account VLFs as well. Too many VLFs will slow down the restore of Transaction log.

Also, all the restores are logged in msdb--> restorefile, restorefilegroup and 
restorehistory tables, so many concurrent sessions will have to write to the same table and if those tables are not maintained (purged) properly, there will be contention/blocking.

As a side note, I have seen blocking especially when you logship 50+ databases with the same restore frequency.

For 3, I will just refer to the most authoritative wait stats repository.