Your drawback might have been attributable to any variety of issues:
In case your system drive is just too full, then that can actually trigger slowness. Deleting caches will not assist you to, as they are going to simply replenish once more.
Previous, incompatible software program — notably these with background processes that run at launch — is also an element.
Nevertheless, one of many causes that Apple excludes a specific mannequin from the supported checklist on a brand new OS is that the “person expertise” is lower than Apple’s requirements. In different phrases, the mannequin is not capable of maintain the efficiency ranges that Apple expects. It could be potential to run Sonoma in your Mac for those who’re doing internet looking, e mail and Workplace apps; however improvement work is one other matter.
If you’re an app developer, then you may’t actually use a 10-year-old laptop. You do not say which mannequin, however the 13″ 2015 had solely 2 cores.