I recently replaced the reverse clutch plates for the same problem.
Mine had worn to the point that I began to hear a slight ticking noise when moving in reverse. This is caused by the thin clutch plates allowing the reverse piston to move too far inward, and it begins to contact the clutch housing. At that point it is mandatory to replace the reverse plates, as the clicking is created by metal-to-metal contact.
when I opened the tranny up, I also found little pieces of the reverse plates in the filter and pan; so it was certainly due.
The delay in shifting is the first sympton of the worn plates. The guys here said the reverse plates were the first to go, the weak link in the tranny.
The good news was, once the tranny was out, I didn't have to go very deep inside, the reverse plates are right in front. The 2nd gear plates are also in the front, and they looked brand new, still had the writing on them.
As stated, it may not get worse for a long time; and until it begins to click or slip, or you find evidence of plate material in the pan/filter, you are probably okay. I would pull the pan and cut the filter open periodically, look for little thin pieces of reddish clutch plate material, about the size of thumbtack heads. Also, keep your ear trained for any sign of a tick,tick, tick when driving (moving) in reverse.
Of course, it's recommended to rebuild the whole tranny if you're paying a shop to pull it out and take it partially apart anyway. In my case, it was working perfectly except for the reverse plates, and I did the work, so I only went as deep as I dared. Now, however, I wish I had done the 4th gear clutches also; that shift is a bit sharp lately. Winter project maybe.

DG