Is my iMac too old and not powerful enough to use for video editing and regular internet use?

Your iMac is plenty powerful to do video & photo editing.

That model (2019, 3.0 GHz, 27" i5 iMac) came with a Fusion drive unless it was upgraded to an SSD at time of purchase. If you have a Fusion drive, that alone could explain the slowness you are experiencing. Find out what kind of drive you have and let us know.

If it's a Fusion drive, the easiest way to improve performance would be to get a good external Thunderbolt 3 (up to 40 Gbps) or USB 3.3 Gen 2 (up to 10 Gbps) drive and install macOS on it and run from the external drive. You could still use the built-in Fusion drive for storage but your Mac will "run" much faster.

That said, as @den.thed already suggested, download & run Etrecheck and post a copy of the report here. There may be other things affecting performance that may need to be addressed. (Use the "Additional Text" button when you post your reply in order to include the full report in your post.)

My 2019 27" iMac works great but it has an SSD and I-9 processor. You may have a slower drive. If so you can use an external Thunderbolt drive to enhance the speed assuming you do not have any other software issues slowing your Mac down. I concur with den.thed to using Etrecheck to assess your system for possible issues.

15.5 is the current, most reliable and securist macOS version.

see > What's new in the updates for macOS Sequoia - Apple Support

The 27" 2019 iMac has decent specs for those requirements and should be adiquate until you are ready to upgrade.

see > Mac - Which Mac is best for me? - Apple

Slowness can be caused by different issues, including anything from lack free space to third-party AV, Protecting, Cleaning junk-ware and VPN's.

EtreCheckPro is an excellent tool for finding and sorting out various problems.

Download and run the free version of EtreCheckPro, from > https://etrecheck.com/en/index.html

Then post back here with your Report, as per > How to use the Add Text Feature When Post… - Apple Community

hieucnguyen wrote:

Yes, this iMac came with a Fusion drive and I've just went and got me a Sandisk extreme pro portable SSD external with read and write speed upto 2000 MB/s.

Macs don't support USB 3.2 Gen 2x2 – so you're not going to see "up to 2000 MB/s" out of that SSD. The USB-C ports on your Mac support USB 3.1 Gen 2 ("up to 1250 MB/s").

You may want to check that your drive has up-to-date firmware.

SanDisk Support – Your Portable SSD May Require a Firmware Update

On the SanDisk Support page, they say "We addressed this firmware issue in the manufacturing process, and we can confirm that the issue is not impacting currently shipping products."

But it wouldn't hurt to check by entering the serial number of your drive into the tool on the SanDisk site.

Just in case part of the problems are due to software together with the slow drive, it may be best to migrate ONLY the user accounts. You should be able to easily install the applications you need, from the App Store or their developer sites.

Before you migrate your user account, system and internet settings, etc. with Migration Assistant make sure you have not installed and run any "cleaning", "optimizing", "speed-up", anti-virus or VPN apps on your Mac.

If you have uninstall them according to the developer's instructions. Then migrate from your Fusion Drive to the new external SSD.

You don't want to bring that crapware over to the new installation.

Good to hear.