Your Mac's CPU, memory and storage
For the duration of your rental, no other customer shares the machine's CPU, memory or storage. Because it's not a virtual machine, another tenant's workload can't compete for resources on the same host.
Dedicated hardware doesn't make every task run at the same speed. Project size, build-cache state, simulator concurrency and network downloads all affect how long your work takes.
Test with a real project before deciding how long to keep it.