Car started revving a lot without gaining much speed

Hi Pros, hope you're doing great. I would appreciate if you could advise me on this unique issue

I bought used the car listed above which I have been driving regularly since February.

This particular issue mentioned below has occurred twice till date months apart.

Yesterday I was caught up in really slow moving traffic for half an hour. After I passed it, the car started revving a lot without gaining much speed, after changing gears. There were no odd mechanical noises. I didn't feel any issue with the clutch or shifting gears. After I drove for about 10-15 minutes in fifth gear the problem went away and everything went back to normal.

This same issue happened a couple of months ago.

Could you please advise what the issue could be.

Kind regards