I have a few seiko with 6R15 movement and all are getting slower over time. Personally I think 6R15 is just entry level movement and seiko is installing some mid range watches with this movement for max profit!
All regular mainline Seikos have mid~low workhorse movements. It's their strategy since the 80s to move the better movements up the price range. For example the 4s15 moved from mainline to the credor brand
