I use both Umobile & Celcom and both got 5G. I use low end 5G android phone (Samsung Galaxy A13 5G) and do some speed test for both telcos.

All I can say from my experience using 5G phone, both telcos provide 2 different speed test.

For example, Umobile tested using Speed test app at Bukit Bintang I got 1015Mbps DL, 89Mbps UL. While Celcom I got like 844Mbps DL, 83Mbps UL.
I've done many tests using both telcos and I can say that Umobile have higher speed than Celcom even though they both are using the same 5G DNB backbone.

Not sure if there something issue with Celcom network on my Galaxy A13, but it supposedly got the same speed like Umobile.