Video editing setup on an iMac & SSD

Hi,

I'm trying to arrange the set up of a new system at work for video editing (plus some After Effects animating). I just want to double check the specs that have been suggested. At a previous company I had a lot of lag watching video stored externally (via a USB 3 connected hard drive). Will the SSD used to store all videos files be fast enough connecting through the Thunderbolt port (USB-C) to watch in real time? On occasion I'll be applying effects or animating in After Effects and will need to view RAM previews too in real time. Or will I need to work off the internal SSD and move data across once finished (not my preference)?

These are the specs:

iMac 27"

CPU: 7th Gen i7 4 core 4.2-4.5Ghz                                         

RAM: 32GB 2400MHz DDR4                                                   

Internal Storage: 512GB PCIe SSD (programs & OS)                                                          

External Storage 1: 4TB SATA SSD USB-C 3.1 (working data)           

External Storage 2: 8TB SATA HDD USB 3.0 (backups)           

Graphics: Radeon Pro 580 8GB DDR5

Many thanks!