Yes, the RAM in your 27-inch iMac can be upgraded. Apple's guidance for that can be found here: Install memory in an iMac - Apple Support
To determine if a RAM upgrade is warranted, you can use the Memory Pressure info displayed by the Activity Monitor app while you use your computer. If the memory pressure graphic often displays in the red, then yes, a RAM increase may be helpful. If it usually displays in the green, then an upgrade is unlikely to be helpful.
Activity Monitor User Guide for Mac - Apple Support
As for the performance (speed) of your iMac, as @tbirdvet has posted an external Thunderbolt drive would speed things up.
If you'd like to learn more about what may be causing your iMac to "struggle", then please run an EtreCheck (free) evaluation of your Mac and post the generated report back here in a reply. EtreCheck is a safe and highly regarded utility application from a trusted developer and respected ASC contributor. The diagnostic report will not include any personal info. It simply gathers specifics about hardware performance and installed software that might be in conflict with the OS.
Please navigate to EtreCheck.com to download the utility. Be sure to Allow Full Disk Access when you install the app. Once you’ve run the app and created your report please post it with your reply to this message.
Please note you must upload the full report. To see how, please click > How to use Add Text when posting… EtreCheck Report.
We’ll use your EtreCheck report to look for the things that may be causing your problem and advise how to correct them.