How to optimize Fusion Drive structure on iMac for faster boot time?

Since you've only got about ½ of your Fusion drive filled a 1 TB external SSD should suffice. I agree with den.thed in that external SSDs from OWC (MacSales.com) would be an excellent choice.

Once you get the external SSD, reformat it to APFS w/GUID partition you can clone your current fusion drive to it according to this user tip: Using Carbon Copy Cloner to create a bootable external SSD of an Intel Mac boot drive.

Yes that looks normal for a Fusion Drive.

If you want faster, startup and run from an external SSD.

see > Use an external SSD as your startup disk … - Apple Community

OWC make several SSD's that are comparable with the LaCie SSD.

see > https://eshop.macsales.com/shop/external-storage#drive-type:ssd,drive-configuration:single-bay-1-drive

The basic question here is "why is it slow?"

Your actual drive transfer speeds likely hold a clue, as the performance numbers for a healthy Fusion Drive are well-understood. Sometimes simply uninstalling unneeded utilities or spotting an underperforming peripheral can restore drive performance. We know some so-called "cleaning" apps can reduce drive speed.

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

An external SSD replaces the COMPLETE Fusion Drive (both the small SSD and internal HD) . DO NOT use an external SSD and attempt to Fuse it to the Mac internal HD!!!!! Follow the instructions in the link below to create a bootable external SSD.

How to setup an external SSD as your startup disk

virajo wrote:

Thank you. So, the SSD external drive is basically replacing the SSD portion of the Fusion Drive?

No you will be using the external SSD instead of the Fusion Drive.

How do I know what to transfer to the external SSD and what size external drive?

You will move everything to the external SSD.

My computer skills are some what limited but not a total neophyte. It is not clear to me what is in a boot drive.

Just follow the directions, at > Use an external SSD as your startup disk … - Apple Community

Well I've already stated my opinion, to me it doesn't make a lot of sense to spend money on an 8 year old computer. You can replace your 2017 with a new M4 based Mac mini. All you need is an external display. Below is a breakdown:

1. Mac mini with 16GB RAM and 1TB SSD $999
2. 27" 4k display, estimate about $350.00

Total $1349.00

VS.

Buy a high quality external SSD such as the https://eshop.macsales.com/shop/owc-envoy-ultra for $379.00

So yes, you will have only spent $379 but you are still left with an obsolete 8 year old computer. IMHO it just makes sense to upgrade, but that is easy for me to say as its not my money!

Like this:

Copy the report as shown in this animated screenshot

and use the Additional Text button to paste the report in your reply.

Then we can examine the report and see if we can determine the cause of the problem.

Yup, if you want faster then it is time to get rid of the 2017 (it's getting close to the end of its useful life IMHO) and get a new Mac. Or you can do as den.thed suggested, taking the Fusion Drive out of the equation and using an external SSD as your startup drive.

As @den.thed already posted, your screenshots look normal for a Fusion drive.

Fusion drives consist of a small SSD coupled with a larger HDD (hard disk drive). What I think you are experiencing is the reality that APFS is not "kind" on HDDs. APFS was designed for SSDs; when used on an HDD, the way APFS works has the effect of slowing down HDD performance, and it gets especially noticeable once the HDD is about 50% full, which is about where your disk is at this point.

If you want to keep this 2017 iMac and improve performance, I also vote for using an external SSD as your boot drive. If you choose to go this route, get an external drive that supports USB 3.1 Generation 2 (up to 10 Gbps) and connect it to one of the USB-C ports on your iMac. You can still use the internal Fusion drive for storage.

I don't see the Etrecheck report posted so we won't know if it's a software problem that can be migrated to a new Mac or not.

So post the Etrecheck report as I suggested previously.

If $$$ is a concern getting a Mac Mini M4 with a 27" or 32" 4k monitor is an excellent choice. I just got a Mini M4 with 16 GB of RAM and 1 TB of storage along with a 32" 4k monitor w/speaker for a total of $1417.65. A comparable configured 24" iMac would be $647 more. You can cut the price down further is you want to use the single speaker in the Mini or earbuds with the Mini.

Bottom line: please first post your Etrecheck report.

The problem that the 1 TB Fusion Drive in that 2017 iMac has a miserly 32 GB of SSD space (vs. 128 GB on the original 1 TB Fusion Drives) will not be transferred to your new computer.

But problems with software installations might be.