Your best option to speed up Lightroom without buying a new computer is to install more RAM in your current device. The 4 gigs of RAM you have is below Adobe's recommended MINIMUM amount of 8 gigs required for Lightroom (https://helpx.adobe.com/lightroom-classic/system-requirements.html). This would explain slow performance of Lightroom on your Mac. The thing about RAM is this: the more you have, the faster your computer can process the edits you are making. Your next best option would be to open up your iMac (I know you don't want to do this!) and install an SSD as the drive where you would then install your operating system. Attaching an external SSD would help a little but not a lot, as the internal 7200 rpm HDD running your OS will be the limiting factor. The combo of an internal SSD running your OS with the addition of maxed out RAM for your computer will change things dramatically. Try downloading MacTracker, a shareware application which will reveal to you all the info about your particular Mac (as well as every other Apple device ever created!), including the maximum amount of RAM you can install on your device. A side note about adding RAM: let's say your current Mac has only two slots for RAM (MacTracker will tell you how many slots you have and what kind of RAM chip you need), and you currently have two 2 gigabyte RAM chips installed. You cannot replace only one of the two with a larger RAM chip, they have to be installed in matching pairs. So if your devices allows a max of 16 gigs and it only has two slots, you would need to throw away the two 2 gig chips and replace them with two 8 gig chips. Or you could install two 4 gig chips for a total of eight gigs of RAM, a lower but perhaps more budget friendly amount of RAM. You simply can't leave a 2 gig chip in place while adding a second chip with a different capacity. However if your computer has 4 slots for RAM, you CAN have differing size RAM chips in the two SETS of slots, i.e. slots one and two can each hold a 2 gig chip, and then slots 3 and 4 can each hold an 8 gig chip, the slot PAIRS need to hold chips of the same size. This scenario would give you a total of 20 gigs of RAM (2 gigs + 2 gigs + 8 gigs + 8 gigs = 20 gigs total). Best of luck to you!
