How can I improve the performance of an older iMac?
The only two things you can realistically do are change the HD to an SSD &amp; upgrade the RAM.

Some iMacs have GPUs that can be upgraded [slightly] but that's not really going to gain you much.

A 1TB SSD can be bought these days for the price your 1TB HD would have cost back then, approx $£€ 100.
According to Everymac that model can take 32GB RAM. I'd recommend at least 16GB these days.  

The SSD alone will put some spring back in its step. The RAM will let it breathe a bit better.