Re-imaging 30 iMacs in computer lab

Re-imaging iMacs:

I cloned an image from unit 1 (specs seen below) via Time-Machine and re-imaged unit 2 (specs seen below) using the clone from unit 1. The system on unit 2 runs, but seems to be a bit sluggish. My actions were actually a practice run to re-image 30 Late model 2015 iMacs (specs seen below). Can anyone help me/direct me on how to do this in a proper manner? It seems that for the image to work correctly, it must come from an identical unit meaning a unit that has the same specs. Also, is Time Machine okay to use (it's pretty simple)?

I have seen tutorials that use Disk Utility or other software.

Unit 1 Specs

iMac (21.5-inch, 2018), Display - Retina 4K display, Processor - 3.6GHz quad-core Intel Core i7, Memory -32GB, Storage - 1TB SSD, Graphics - Radeon Pro 560 with 4GB of VRAM​

Unit 2 Specs

iMac (21.5-inch, 2017), Display 21.5-inch, Processor 2.3GHz dual‑core Intel Core i5

Memory 2.3GHz 8GB of 2133MHz DDR4 memory, Storage1TB (5400-rpm) hard drive, Graphics Intel Iris Plus Graphics 640

Unit 3 Specs

iMac (21.5-inch, Late 2015), Display 21.5-inch (diagonal) LED-backlit display with IPS technology,

Processor 1.6GHz dual‑core Intel Core i5, Memory 8GB of 1867MHz LPDDR3, Storage 1.6GHz 1TB (5400-rpm) hard drive, Graphics 1.6GHz Intel HD Graphics 6000

Thank you in advance for your time concerning this matter.

A. Bryant