How do I speed up my 2017 iMac?

Those base model 21.5" iMac have a really slow 5400 rpm internal HDD.

The best way to get more use out of it, is to run it from an external SSD startup disk.

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

The two things you can do (in priority order):

1. As den.thed advises, replace the HDD with a SSD;
2. Increase the RAM from 8GB to 16GB (this iMac can expand up to 32GB but you likely don't need that much). However, the RAM slots are not user accessible on this iMac18,1 model and this upgrade requires an Apple Authorized Service Provider - but you might want to seek one out anyway instead of replacing and cloning the drive yourself.

You also may want to simply update to a new Mac. The current Macs are DRAMATICALLY faster , so much so you will really be pretty shocked. Why, beginning in 2021 all MCs now come with SSDs which are drastically faster than traditional hard drives like your 2017 has.

Just good for thought.