ADVICE? external SSD for OS speed upgrade on 2019 iMac?

Your iMac (27" 2019, Intel i9) has two USB 3.2 Gen 2 (USB-C) ports that support 10Gbps transfer speed. I have the same model and am using a number of external USB 3.2 Gen 2 SSDs and the performance is great.

Regarding USB4, you have to be careful. USB4 by itself is not backward-compatible with earlier versions of USB. Check the drive specs to make sure the drive also supports USB 3/2/1 or at least says it is backward compatible.

Also keep in mind that the USB speed is always limited by the slowest device, so even if a USB4 drive supports 40Gbps and it's also compatible with USB 3.2 Gen 2 and you connect it to your 2019 iMac's USB-C port you will only get 10Gbps speed. And if you connect it to one of your 2019 iMac's USB-A ports you will only get 5Gbps speed.

Maybe before spending money......exactly what storage configuration do you have now?

That model never shipped with mech HD-only storage, the big bottleneck in most "slow iMac" complaints here. Entry point for your model was a 1TB Fusion drive, and larger Fusion system plus large-capacity Flash storage were options.

A healthy Fusion Drive will outperform most external USB SSD setups. Data I saved from late 27-inch iMac Fusion performance reported here show Writes at 600-900 MB/sec and Reads up to 2000MB/sec. The external USB3 SSD option can be as a slow as 400MB/sec for both speeds depending on the enclosure and SSD used). They seldom can exceed ~900MB/sec in the real world.

Fusion drives are not bulletproof and can have issues when someone diddles with the OS.. Fortunately, a "split" Fusion drive is user-fixable.

The factory optional SSD-only config is the benchmark to meet or beat. My 2017 iMac 5K with the factory 1TB SSD does writes from 2200-2600MB/sec and Reads up to 2900MB/sec. Thunderbolt external can easily match those speeds.

Speed issues can be as simple as removing unneeded software. Before spending money, give us a peek at your current storage config and its performance. Please post an EtreCheck report. We can quickly and within the limitations of these forums help you determine what issues are at play without our playing a protracted game of "20 Questions" with you that could go on for days. 

EtreCheck Pro is available here: https://etrecheck.com/index

The free version will do nicely for this purpose, although the app is worthy of our financial support.

We can see hard data about drive performance, software interferences, and RAM usage. Etrecheck is a reporting app, not a "fix-it” app, expressly for displaying information in these forums to help us help you remotely. It will not reveal any personal or secure information.

That gives us a better opportunity to a specific recommendation for your individual situation.

Please see this excellent user tip on how to post long text reports like EtreCheck's into a forum response:

How to use the Add Text Feature When Post… - Apple Community

Your Mac does not support USB 3.2 Gen 2 x 2, or USB4 anything.

The fastest external SSDs would be Thunderbolt 3 / PCIe NVMe ones. Next fastest would be USB-C (USB 3.1 Gen 2) / PCIe NVMe ones (including USB 3.2 Gen 2 x 3 ones forced to fall back to the lower speed).

Slowest would be SATA SSDs.

@BDAqua wrote:

I'd recommend Thunderbolt, I use these on my 2019 iMac...

https://eshop.macsales.com/item/OWC/TB3ENVPFX02/

I agree that Thunderbolt yields the fastest transfer. The reason I don't typically recommend it is due to the significantly higher cost of Thunderbolt peripherals compared to USB.

on visiting the OWC (MacSales.com) site. Contact their customer support and see which of their several models they recommend for your particular Mac model, workflow requirements and budget restraints. OWC is considered the premier 3rd party hardware supplier for Macs by most of the experienced contributors here.

I'd recommend Thunderbolt, I use these on my 2019 iMac...

https://eshop.macsales.com/item/OWC/TB3ENVPFX02/

Visit the OWC/macsales website and explore the options they provide for external Mac storage.

They know Macs inside and out and are easy to work with. (I'm not affiliated, just a satisfied customer.)

OWC External Hard Drives for Mac and PC

I just purchased a Crucial X9 Pro SSD that is USB-C 3.2 Gen 2 and rated at 1050 MB/s read (and write). That drive flies, but in a departure from earlier Crucial X-series drives, it no longer ships with a USB-A to USB-C dongle. Just a USB-C 3.2 Gen 2 rated cable.

Yes, booting from a big fast External SSD will speed things up.

OneDrive & Dropbox are slowing thing a bit.

Here's my 2019 21.5" iMac booting from external Thunderbolt3 2 TB SSD...

Personally, I wouldn't worry about the USB ports speed.Two things mainly affect speed of operation having sufficient RAM installed (typically 16 GB or more) and using an SSD as the startup disk. If your Mac has an older, slower mechanical drive, using the SSD will speed things up enormously.

Here's my results for the 2019 iMac 1 TB Fusion drive... 1st run then 4h consecutive run where Flash SSD is full & using HDD part...