Connectivity and speeds are good. I have an AIS 5G plan (postpaid) and usually get 200-300mbps down / 50mbps up. As with all cellular connectivity, there can be localized dead spots, weather impacts (especially heavy rain), and just random oddness.

One thing to look out for is that many of the cheaper SIMs have speed caps in addition to data caps: a max speed of 10mbps or similar. Make sure you get a SIM and plan that does not have speed restrictions, which usually means not one of the tourist SIMs from the airport.