Not to be overly pedantic, but Apple didn't release any quad-core MacBook Airs in 2015.(see https://support.apple.com/kb/sp714?locale=en_US) Intel didn't even have quad-core ULV CPUs available until late 2017-2018.

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.)