The cable might have stretched when it was being pulled, but once it stops being pulled, it will not continue to elongate. It is common for new bikes to experience a little cable stretch over the first month or two of use, then it is usually done.

For a cable that is a fixed length (for you maybe now a little bit longer fixed length than before), slack/tension should be able to be adjusted to recover good shifting.

A bunch of related things to check for:

Did the housing get pulled partly out of its housing stops? Or are there ferrules that are not all the way on? (Or did there used to be loose ferrules but they closed up now?)

In an extreme case (probably not here), the derailleur or hanger could have gotten bent, and that misalignment would cause poor shifting that could not be tuned out by any amount of barrel-shifter twiddling.

If the only problem left is slower changes in one direction, it's probably just cable friction. New cable/housing would solve that, and it can never hurt, especially when you're unsure how old your cables are.

But in the meantime try dropping chain lube or other light oil into all entry points of your housing.

Shift to a big cog, then put slack in the cable by shifting the other way (with the bike stationary so the chain remains on the big cog). Then pull each segment of housing out of its stops, and drop 5-10 drops of oil in each end. Slide the housing back and forth on the cable to distribute. You might feel the housing start to slide easier as the oil works around, which would indicate that cable friction was indeed your problem.

If you can't get smooth enough cable/housing that way, it might be the cable is so old that there is some fraying somewhere (inside the housing?), in which case you definitely need to replace.