For SingTel TV Go, why not consider Chromecast dongle if you do not like Mi Box S? You do not need to buy the more expensive Chromecast Ultra, just the normal Chromecast dongle will do.
In fact my old first generation Chromecast dongle bought from SingTel shop many years ago at S$35 still works fine for all the apps like Toggle, Starhub Go, SingTel TV Go, YouTube, etc.
And Mi Box S is only struggling a bit if you set the interface resolution to 4K, set it to 1080p and it will be very smooth. Anyway, none of the local entertainment solution, be it SingTel TV, StarHub Fibre TV, SingTel TV Go, SingTel Cast, Toggle, supports 4K. In fact for YouTube 4K and Netflix 4K, Mi Box S performs pretty well.
Ref: read the reviews below
https://androidpctv.com/review-xiaomi-mi-box-s/
Lon.tv review
https://youtu.be/qTQtVgo6aWI
Chigz Tech review
https://youtu.be/mHMeN-M9xSE
Tech4All review — 5 reasons why buy Mi Box S
https://youtu.be/uzjC64FKzWo
Negative reviews from Tech Hive
https://www.techhive.com/article/3319662/xiaomi-mi-box-s-review.html
For 1080p, even boxes based on the lower performance CPU like Allwinner H3, Amlogic S905W, Rockchip RK3239, perform very well. I have two 2GB/16GB low end boxes based on S905W and RK3239 and they are okay when it comes to 720p/1080p streaming applications. I have tried Orange Pi H3 board with 1GB memory as well and it is actually fine even with Google services enabled.
I have another low end box with S905L professor and 1GB memory, which is of lower spec than Mi Box S (similar CPU spec, lower memory, similar good dual band WiFi performance), and it performs very well (no Google services) on 720p/1080p Streaming applications.