I've also noticed a drop in Speed over the past year. When I first signed up the previous year, it was pretty fast - on LTE, I could consistently hit 130Mbps at many outdoor places (so it's not using the indoor cells). Now it's much lower.
I've also noticed that network quality seems to coincide with the increasing/decreasing MVNOs (ie, additional users = additional load on the network). With Vivifi and ZYM being quite aggressive now, it could make Singtel's network a little more congested. It could in some ways explain Starhub's better speeds now - they have only one MVNO left under them iirc, whereas Singtel seems to be aquiring, and M1 has a lot. The "unlimited data" bundles don't help matters, even if throttled.
However, speedtests are not everything - most of the time we can still work reasonably well even with "30Mbps" speeds, so we won't really feel it. We will feel the loss of coverage much more (and that's my push from M1 based Changi Mobile).
Again, it's all location based - so the current advise is to try and see if it works for you. M1 may be poor for many but could be fantastic for some. If you are typically confined to a few fixed places 90% of the time (work, home, school, place of worship, etc...) it could work well. If your job requires you to go to various locations across the island and hence no fixed locations, it could be an issue for some networks over others. There's no perfect 100% coverage network, but some do better than others. For me, it's ST and SH that seem to cover 100% of my use cases, but when it comes to a work line, I need to be judicious (since I call patients from that line too).
