my macbook pro mid 2012 become very slow after updating to mojove boot up time is 2min and 30 sec how can i increase its boot time speed my harddrive is empty

If the computer was not ordered with a different drive option the the entry level, or has not been treated to an aftermarket faster drive solution, it is your hard drive. Apple uses a slow, 5400 RPM drive rated at SATA 3GB/sec in a computer with a fast SATA 6GB/sec drive bus. Adding RAM will make zero difference in your boot time if you still have the slow factory hard drive.

I have the same computer and upgraded it to a solid-state drive at home about three years ago. To demonstrate how RAM and then a fast SSD affect app load times. I have this data from my MBP13 2012:

Base system as shipped:

4GB RAM and slow SATA 3GBps 5400rpm hard drive: Office 2008 and Photoshop Elements 12 took 15-18 seconds to be ready to use.

First upgrade, RAM:

8GB RAM and slow SATA 3GBps 5400rpm hard drive: Office and Photoshop Elements took 15-18 seconds to be ready to use.

Second upgrade, inexpensive solid-state drive

8GB RAM and fast SATA 6GBps SSD: Office and Photoshop Elements take under 4 seconds to be ready to use.

The boot time is now quite fast but I've not checked in lately

Part of the problem is just the age of the machine.

Check the health of the hard drive with http://www.binaryfruit.com/ DriveDX.

If the drive is showing failure imminent, bring it in to a service center and see if they will change your drive to a SSD.