Adding RAM (iMac Retina 5k, 27 inch, 2017)

That is normal when opening a large image from a slower system and/or an external drive.

In your case, an 8GB (4GB x 2) or 16GB (8GB x2) RAM upgrade would help, if your system is running low on RAM.

see > Check if your Mac needs more RAM in Activity Monitor - Apple Support

Crucial no longer offers RAM modules for that iMac and you should beware of other vendors selling Crucial RAM.

Go to OWC and order the 8GB (4GB x 2) or 16GB (8GB x2) kit and install those modules into the two empty slots.

https://eshop.macsales.com/shop/memory/imac-2017-27-inch

The problem you describe - photos opening blurred then popping into focus - is common and is not something just in Photos.

8GB RAM is bare bones, enough to run a Mac on Ventura and do very basic tasks. No surprise that you may see some issues in the Photos app and others that require significant system resources.

You would do well to upgrade to at least 16GB RAM. OWC & Crucial are both good for Mac RAM but recently Crucial has not had RAM available for 2017 iMacs. OWC does, 16GB is about $32.

Your 8GB iMac probably has 2x4GB RAM modules. There are 4 RAM slots, so you can add more RAM. Adding 16GB to your Mac will bring it up to 24GB which will be plenty for work with most photos.

The redraw time can also depend on your video hardware. The base GPU for that model was the AMD Radeon Pro 570 with 4GB VRAM. Options were a Radeon Pro 575 with 4G VRAM, or the Radeon Pro 580 with 8GB VRAM.

My 2017 5K has the 4GB 575 option and 16GB RAM (8GB Apple; 8GB OWC). I seldom encounter redraw delays even on 3-60MB Photoshop (.psd) files..

VRAM is soldered to the logic board and cannot be upgraded. "About this Mac..." will show your video hardware config.

OWC's page for the 2017 iMac 5K: https://eshop.macsales.com/shop/memory/imac-2017-27-inch

IMHO, anything over 32GB RAM in that model overkill for the typical user. I thought I would go from 16 to 32 but have not needed more. Still happy with 16GB.

What storage option do you have? If you have Apple's Fusion Drive, it may be productive to post a system config report so we can see if it is underperforming. Fusion has proved less than perfect over time, so knowing your drive performance scores can help us decide if an SSD option would benefit you.

If you want a data-driven evaluation in this setting where we can neither see nor touch your computer, 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 issues and interferences, and RAM usage. Etrecheck is the development of a long-serving and trusted ASC contributor. It 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.

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 8GB iMac probably has 2x4GB RAM modules.

For clarity, "2x4GB RAM modules" indicates two 4GB modules. The 2017 iMac has 4 memory slots, so two would already be occupied by the existing modules, leaving two more slots where you can add RAM.

RAM modules should be added in pairs, so in order to add 16GB you would get two 8GB RAM modules, not just one 16GB module.

It sounds almost as if you are dealing with a combination of progressive JPEG files, and loading taking a while. You'll often find progressive JPEGs on Web sites. The idea is that if loading is going to take a while, it's nice to display something (even if it's low-quality) up-front.

https://www.thewebmaster.com/progressive-jpegs/

Think_differently1 wrote:

Thank you both.

You're welcome.

Given the difference in cost is under US$ 60, is there any reason not to just go with 64GB?

The upgrade that you choose is totally up to you.

Will that be noticeably different from 40GB (assuming I were to add 2 x 16GB to the 2 x 4GB already there).

Think about it.

Adding 8GB (4GB x 2) is two times the RAM that you currently have.

Adding 16GB (8GB x 2) is three times the RAM that you currently have.

Also would 64GB mean there is any advantage that the RAM in each of the 4 slots would be the same?

No there is really no advantage in replacing all 4 modules.

IMHO it is a waste of money to max out the RAM in that older vintage iMac this late in the game.

Your time and resources would be better spent on finding a new Mac to replace that iMac.

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

Re: “If you have Apple's Fusion Drive, it may be productive to post a system config report so we can see if it is underperforming.”

If it is a 1 TB Fusion Drive, it would have only 32 GB of SSD space (vs. 128 GB in the original 1 TB Fusion Drives). In that case, even if the drive was not failing, starting up from an external SSD might well be in order.