This will work but keeping the data synced between the two schemas will be difficult. It’s much easier to keep everything in a single schema.
What’s the cause of the month end update statement taking so long? Are the tables indexed correctly? Are there indexes which can be removed? Are the disks queueing to much?
Your will probably be better served in the long run to try and fix the performance problem on the existing system before you try and create a duplicate which will make the management of the data much more complex.