Yes it has been the case. They were still using 16 or 32 bit mission computers when virtually every street hawker had access to a 64-bit quad-core/octa-core smartphones.
But that's gonna change in F-35 block 4 upgrades. There are so much drastic changes that it would virtually be equivalent to a top notch crypto currency rig. The requirements of having feeds from multiple assets simultaneously and fusing them into a single realtime display output while also relaying similar amounts of data to multiple assets too. So 5th gen fighters with advanced datalinks, sensor/data fusion, complete 360 degree situational awareness in 3D environment, controlling multiple companion drones as well as sharing realtime video feeds with various assets have now pushed the boundaries of computing power in modern 5th gen fighters and upcoming 6th gen fighters. We would find upcoming blocks of fighters as flying huge a$$ GPUs