short story about a man who wakes to find that time is running very slowly, but then time speeds up

I'm looking for a short story

"Common Time", a novelette by James Blish, published in Science Fiction Quarterly, August 1953, available at the Internet Archive. The story has its own Wikipedia page.

about a man who wakes (maybe on a spaceship) after some accident

It's a spaceship all right. There was no accident, but it's an experimental faster-than-light ship, and no one knows just what to expect:

Otherwise everything seemed normal. The DFC-3 had crossed over into interstellar velocity, and he was still alive, and the ship was still functioning. The ship should at this moment be traveling at 22.4 times the speed of light—a neat 4,257,000 miles per second.

Somehow Garrard did not doubt that it was. On both previous tries, the ships had whiffed away toward Alpha Centauri at the proper moment when the overdrive should have cut in; and the split second of residual image after they had vanished, subjected to spectroscopy, showed a Doppler shift which tallied with the acceleration predicted for that moment by Haertel.

The trouble was not that Brown and Cellini hadn't gotten away in good order. It was simply that neither of them had ever been heard from again.

to find that time is running very slowly,

Garrard is counting off seconds in his head while watching the (analog!) clock:

The second hand began a slow, preliminary quivering as the calendar's innards began to apply power to it. Seventy-hundred-forty-one, seventy-hundred-forty-two, seventy-hundred-forty-three . . .

At the count of 7,058 the second hand began the jump to the next graduation. it took it several apparent minutes to get across the tiny distance, and several more to come completely to rest. Later still, the sound came to him:

pock.

In a fever of thought, but without any real physical agitation, his mind began to manipulate the figures. Since it took him longer to count an individual number as the number became larger, the interval between the two calendar ticks probably was closer to 7,200 seconds than to 7,058. Figuring backward brought him quickly to the equivalence he wanted:

One second in ship-time was two hours in Garrard-time.

Had he really been counting for what was, for him, two whole hours? There seemed to be no doubt about it. It looked like a long trip ahead.

Just how long it was going to be struck him with sudden force. Time had been slowed for him by a factor of 7200. He would get to Alpha Centauri in just 72,000 months.

Which was—

Six thousand years!

so he believes he has all the time in the world to think about puzzles,

There now, that was very good; there had been two Garrard-hours which he had passed with virtually no difficulty of any kind, and without being especially conscious of their passage. If he could really settle down and become used to this kind of scheduling, the trip might not be as bad as he had first feared. Sleep would take immense bites out of it; and during the waking periods he could put in one hell of a lot of creative thinking. During a single day of ship time, Garrard could get in more thinking than any philosopher of Earth could have managed during an entire lifetime. Garrard could, if he disciplined himself sufficiently, devote his mind for a century to running down the consequences of a single thought, down to the last detail, and still have millennia left to go on to the next thought. What panoplies of pure reason could he not have assembled by the time 6,000 years had gone by? With sufficient concentration, he might come up with the solution to the Problem of Evil between breakfast and dinner of a single ship's day, and in a ship's month might put his finger on the First Cause!

but then time starts to speed up

Pock.

Not that Garrard was sanguine enough to expect that he would remain logical or even sane throughout the trip. The vista was still grim, in much of its detail. But the opportunities, too, were there. He felt a momentary regret that it hadn't been Haertel, rather than himself, who had been given such an opportunity—

Pock.

—for the old man could certainly have made better use of it than Garrard could. The situation demanded someone trained in the highest rigors of mathematics to be put to the best conceivable use. Still and all Garrard began to feel—

Pock.

—that he would give a good account of himself, and it tickled him to realize that (as long as he held onto his essential sanity) he would return—

Pock.

—to Earth after ten Earth months with knowledge centuries advanced beyond anything—

Pock.

—that Haertel knew, or that anyone could know—

Pock.

—who had to work within a normal lifetime. Pck. The whole prospect tickled him. Pck. Even the clock tick seemed more cheerful. Pck. He felt fairly safe now Pck in disregarding his drilled-in command Pck against moving.Pck, since in any Pck event he Pck had already Pck moved Pck without Pck being Pck harmed Pck Pck Pck Pck Pck pckpckpckpckpckpckpck. . . .

so he doesn't after all.

Good-bye, vast ethical systems which would dwarf the Greeks. Good-bye, calculuses aeons advanced beyond the spinor calculus of Dirac. Good-bye, cosmologies by Garrard which would allot the Almighty a job as third-assistant-waterboy in an n-dimensional backfield.

Good-bye, also, to a project he had once tried to undertake in college—to describe and count the positions of love, of which, according to under-the-counter myth, there were supposed to be at least forty eight. Garrard had never been able to carry his tally beyond twenty, and he had just lost what was probably his last opportunity to try again.

Time continues to speed up for Garrard:

Pock.

Garrard looked up in surprise. The familiar noise, this time, had been the hour-hand jumping one unit. The minute-hand was already sweeping past the half-hour. The second-hand was whirling like a propellor—and while he watched it, it speeded up to complete invisibility—

Pock.

Another hour. The half-hour already passed. Pock. Another hour. Pock. Another. *Pock. Pock. Pock, Pock, Pock, Pock, pck-pck-pck-pck-pckpckpckpck. . . .

The hands of the calendar swirled toward invisibility as time ran away with Garrard.

I just read a short story by R.A. Lafferty about a man that wakes up and time is going very slowly, "The Six Fingers of Time"; at the end times returns to normal