questions about stock tranny w/ 157k on it...

[Oaktown]

i have 157k on my stock tranny. it doesnt slip, but it is a little slow when changing gears...what does this mean? could a shift kit help firm up the shifts? It does good when i shift it manually...any input appreciated...im in the early stages of fixing this problem...i have reciently changed the filter and fluid to see if that helped at all, but it is pretty much the same...