This kind of issue happen mostly to older devices. Maybe you can try:
1) Restart and open the application
2) Clear cache and storage
3) Make sure the application is up to date
4) Make sure you have sufficient RAM
5) Try at different Wi-Fi location or different Telco.
Is the sluggish happen only to Instagram Reel? What about other application like opening a video on chat messenger, sharing a video on chat messenger, open a video on Youtube/ Twitter/ Instagram?
This kind of problem is actually due to the slow internet speeds, outdated application version and low RAM. I am not sure your devices can change the virtual RAM (RAM Plus) or not, if yes you can try put a lower/ higher RAM other than the default settings.
If still no luck, you can try to contact Samsung Support through chat or email. Guess the only thing they will ask you to do is to reset or clear your phone storage.
Same issue on my A53. What I found very weird is that the A33 with the same chipset does not have this issue. I have already reported it on Instagram and I doubt it's gonna make any difference. Also reported on Samsung Members app today.