I'm using a MacBook Air 2015 and it's basically still a fast, perfectly usable machine. Quad core i7, 8GB RAM, SSD. However, I wish I could replace the battery, I wish the SSD was more easily upgradeable, and it would be nice to double the RAM.
I only comment because the jump between dual & quad core is a pretty big one, as is the presumed jump to newer chips with 6, 8, and 16 core processors in modern machines.
Dual core has remained surprisingly usable for light tasking though. Whether it is a well-loved MacBook or a battle-scarred ThinkPad from that era, Intel's dual core CPUs have held up well (even if it means they didn't feel the need to push their CPU design as much in the subsequent years.)
https://www.ifixit.com/Guide/MacBook+Air+13-Inch+Early+2015+...