There are a few points to consider.
Adobe recommend that InDesign requires a SSD drive and a minimum 4GB RAM, but 16GB is preferable.
Your iMac has most likely a spinning disk hard drive, which is going to be very slow, and with only 8GB memory available for Monterey and InDesign to run in then InDesign will have to use virtual memory on your hard drive in order to operate.
The combination of a slow internal hard drive and limited RAM are the principle reasons for the poor performance you are now experiencing.
At wake-up from sleep Monterey runs dozens of background processes, wake-up from power-off is even more demanding with hundreds of background processes running, this is why performance improves after the Mac has been switched on for a while. Gradually those background processes complete and this releases more RAM memory for your application to use.
Lastly, whenever you upgrade a macOS, Apple’s Spotlight service needs to build a fresh index of the disk, and if you use Time Machine to back-up your system as well then Time Machine also needs to scan the entire disk to build a new backup.
Both those processes will take up to 48hrs of continuous on-time to complete and until those two initial processes have finished running your Mac will run extremely slowly.
As GreeniusGenius asked in his reply “Do you have an SSD?”, from your description of the problem I suspect your answer is no.
The ideal solution would be for you to take your iMac to an Apple store and have them replace the internal spinning-disk hard drive with an SSD and increase the RAM to 16GB, this would improve performance markedly.
Being realistic, the cost to have this done in an Apple store would be close to two thirds the price of a new iMac with SSD and 16GB memory already installed, and certainly more that the current value of the Mac.
The most cost-effective thing you could do would be to connect a comparatively inexpensive external SSD enclosure to one of the thunderbolt ports and move your macOS and applications over to that external SSD.
Monterey should boot faster, applications will launch more quickly and your applications will be more responsive when using virtual memory on the external SSD than when using virtual memory on the old internal disk.
Overall performance should return approximately to pre-Monterey levels, although not such an improvement as would be gained if you upgraded the internal disk to an SSD and increased the RAM to 16GB.
If you found that after installing an external SSD with the macOS and your apps loaded does not improve performance as much as you would like then at least you can use that SSD as normal storage, or as a Time Machine backup drive on your existing or newer iMac.
The final points to consider include what other applications do you have installed that may be running their own start-up background processes, consuming RAM and processor time, and any third-party anti-virus software you may have installed, both of which will cause the Mac to slow down.
If you have not already done so, run a system check on your Mac using the free version of etrecheck and post the results back here for the experienced Mac experts to comment on. Etrecheck is not installed as such, it runs directly from the download and can be simply deleted afterwards. Be sure to grant permission for etrecheck to have full access to your hard drive otherwise the test results will be meaningless. Etrecheck does not share or record private data and is safe to use.
The free version of etrecheck is all that is required to generate the report, the paid-for version is unnecessary for the use suggested here.
https://etrecheck.com/
Hopefully some of the above may prove useful.
Will.
