Settings > Software update > Download and Install > Software information
Same problem! I've just moved from my Galaxy Note9 (Exynos) to a S21 Ultra (also Exynos) and my 4G download speeds have gone from ~25Mb to ~8Mb. In the late evenings when the awful network in my area isn't congested, I used to get around 50Mb, now I get 15Mb. Such a massive noticable downgrade!
On EE and 0-1 bar outside in area that coverage checker states EXCELLENT. Mate has the same phone and the same issue in my area. Done usual reset network settings, sim out clean, try another network and switch back, aeroplane on then off. Nothing improves it and phone cannot do mobile data. Works fine in other areas so I have raised an issue with EE as I suspect it's a coverage issue and needs investigating. Also, had the same weak signal on my Pixel 4 XL before I swapped it.
i use Three and EE
With Three i get 800mbps download using 5G
EE i get 200mbps download using 4G+ payg
don't use vodafone for 5G neither O2 5g as they are still slow