Anyone know the title of a sci-fi short story
"Whatever Became of the McGowans?", a novelette by Michael G. Coney, first published in Galaxy Magazine, May 1970, available at the Internet Archive.
which ends with the protagonist growing roots on his feet and sinking them into soil, finding peace.
At last the tendrils on his feet probed moisture below the surface and the fluid rose into his body, satisfying a craving that had possessed him for days. Once again contentment stole through his senses and he felt his heart slow until it became an occasional spasmodic flicker.
However it doesn't end there: the man and his family end up in a hospital on Earth:
"I'm glad to see you're awake. Now, first of all, I want you to know that your wife and child are all right. You are in the Earth Rehabilitation Center and I am talking to you through this machine because for the time being you will not understand normal speech. My name is Dr. Svenson and from time to time I sit beside you—on the chair you will see beside your bed."
You added some details in a comment:
Pretty sure it was 70s or 80s, and in an anthology.
If it was in English, it must have been World's Best Science Fiction 1971, edited by Donald A. Wollheim and Terry Carr.
The character gradually slows down
Yes, it happens gradually. In the beginning:
"Richard, what on Earth are you doing?"
Sandra was standing at the barn door, her brown hair a halo in the sunlight. But her expression was ominous.
"Just seeing to the harvester. What's the trouble?"
"Do you know what the time is?"
"It's past two, we haven't had lunch. What have you been doing?"
Bewildered, Richard shoved up his sleeve with the back of his hand to keep the oil from his sweater. He studied his watch. She was right. Its hands stood at two-fifteen. Had he fallen asleep, dozed, daydreamed, idled between chores? He couldn't remember doing so.
In the end:
Richard saw the chair and saw also that it was in almost constant jiggling motion. From time to time he thought he could make out a sitting, semi-transparent figure.
"I can't see you properly," Richard addressed the phantom.
A slow fear was beginning to flow through him, driving away his lethargy.
"That is because I am not always here," replied the tape recorder. "Time has accelerated for you. When you spoke I had time to replay your remark at high speed, then record my reply and play it back to you at reduced speed—but I don't suppose you noticed any time lag."
and I think stops eating
He wandered back indoors. The tray was on the kitchen table, where he had left it. How long was it since he had last eaten?
Three days? Four? He couldn't remember. The time lapse bothered him. Deciding that he would in any event, have a good substantial supper later tonight, he drank the remains of his glass of home-made wine, then followed it with a glass of cold water. Presently, he began to feel hungry, just a little.
In another comment you said that this asker seemed to be looking for the same story. A couple of points from that other query:
Some time back in the 80s I read a SF short where humans were colonizing a new planet.
Yes, the planet is called Jade:
Several hundred other customers of the Jade Exploitation Company were, he believed, scattered among the valleys and along the coast of the planet's only continent. Not that their presence made much difference—the distances separating homesteads made visiting impractical. Each settler had his own place to look after.
I seem to recall that some folks had started to disappear, and there may have been an investigation.
Yes, the colonists on the neighboring farm, the McGowans of the title, had gone missing:
The wire fence dividing his spread from the McGowans was down. The stainless thread trailed a random silvery path through the grass. As the McGowans were no longer around he hadn't bothered to repair the barrier and noticed, with a pleasurable thrill of dishonesty, that the grass grew even better on the other side. At harvesting time he would reap their grass as well as his own and profit by the proceeds—it would save the crop's going to waste. If the McGowans ever came back he could always pay them for it, less a deduction for labor.
In front of the McGowan household a small clump of trees provided a cool and tempting patch of shade. He sat down, his back against the largest bale, and regarded the house. It was bigger than his own and in a good state of repair, despite at least two years' disuse.