model is a MacBook Pro, Intel Core i5-4278U @ 2.60GHz, model A1502 (EMC 2875), Retina Mid-2014 13" with an embedded SSD

Unfortunately, I don't have any first-hand experience with this device. But, I do own the following potato; an Acer laptop with Intel Celeron, 2GB of RAM and no SSD from 2014-2016. And while the experience is pretty bad (on Zorin OS lite), it does its job as a backup laptop every once in a while. Compared to this potato, your device should be a lot more capable. So, either your expectations are off. Or, there's something legitimately wrong with the hardware found on the device. Have you done any benchmarks to see if they work as expected?

Some mac OS users mean this company deliberately slows down old computers so users feel compelled to buy something newer. Can it be that’s why this notebook is so slow?

Slowing down of devices is AFAIK done (un)intentionally through updates. In a lot of cases either some functionality is removed post release for security reasons or (through technological advancements) more is expected from your average device and hence older devices fail to compete. I don't think you should suspect anything else. Nonetheless, as previously alluded to, maybe some hardware failure is the cause.

I didn’t do anything fancy to install xubuntu, just used the whole space to install from a usb stick so I wonder if some residual software is still present.

This description of the installation seems fine. If it makes you feel better, you could consider deleting all partitions through something like GParted. But, usually, no residual software should be left behind.