A little late to the question, but Paul is incorrect in the original question as he states that 24GB of RAM cannot be improved on. This model can actually handle 32GB of RAM (4 x 8GB) 204-pin PC3-12800 1600 MHz DDR3 SODIMM.
In 2024 the amazing OpenCore Legacy Patcher can take this computer to macOS 14 Sonoma, but my preference would be macOS 13 Ventura as Sonoma has some fairly heavyweight updates built into it and it shows in slow operation.
It's generally accepted that any iMac from Late 2012 with a spinning HD inside (HD SATA or Fusion Drive) will suffer badly from speed loss due to the new Apple drive formatting called APFS. It seem APFS is designed for SSD or M.2/NVMe and possibly results in high levels of data fragmentation on the HD and the HD part of a Fusion drive. Whatever the reason, upgrading to a SATA SSD or better still, an NVMe drive with a small adapter, will give you a massive speed boost.