Is scheduler is a CPU hog?

I run Scheduler for a site with about 15,000 invoices. Scheduler runs weekly and because there’s no way to set the time it should start per site, the default is Monday(12 am).

I’ve come to realise that between the hours’ of 12 am and 4 am, I’ll run out of memory(2Corse, 4Gig, Ubuntu 18.04).

This is worrisome and I’m wondering why this happens and if there’s a fix besides disabling Scheduler.