As the OP's wheel has cartridge bearings and doesn't have a threaded (thus adjustable preload) axle design I suspect the bearings are getting worn. A new (or in good condition) cartridge bearing will roll smoothly and easily even when the preload is way too high. That same bearing won't spin nicely when well worn and given the same way high preload. The method of checking a bearing's spin that we most all can do is not truly duplicating the conditions when secured in the frame/fork, we spin the axle with our fingers, but with no QR loading.

If this wheel is treasured and if the lack on in fork spin is aggravating enough, the cost to replace the bearings is a bargain. Andy