In reply to Removed Usermick.
How old is the bike/cables/derailleur? Sounds like stiff(rusted) cables to me. I'd start with those. Replace if possible, or if you want to be frugal, get at the bits that run inside the outers (by undoing them at the derailleur end and fiddling the outers out of the way). Rub down the bits of the inner that run inside the outers lightly with some wire wool, oil liberally before putting things back in place. Don't pull the inner cable out of the outer bit if they are in anything less than perfect condition, you may well never be able to get them back in without damaging them further. Might not be the solution, but probably will improve things.
Another thing that might be an issue is the derailleur itself. If that is the case releasing from the cable, spraying liberally with gt85 or similar and working vigorously back and forth to each end of the movement to free it up a little might help. But if this is the issue it's probably the springs weakening over time (always leave in low at front and high at back to leave springs relaxed to look after your derailleurs), in which case replacement of the derailleur is the way forward.