Question for other Merchandise Managers

I've gotten hired as a merch manager and I enjoy the job, however, my SM has begun to question my speed regarding putting out u-boats. I got one out in roughly 3 hours the other day (yes I am aware that's an extraordinarily long time..)

I am having a hard time getting my timing up better especially when I have to stop often to help out a cashier and then get back to what I was doing. I know I can be faster but I'm combatting that plus age (I'm mid-40s) along with a knee problem and arthritis in my hands. It doesn't stop me but I am finding it more difficult.

Any tips would be appreciated - I mostly do food/candy, chemical and housewares stocking.