I do a similar travel distance (between 213km - 230km by car on the highway) monthly minimum between Amsterdam - Brussels taking the inter-regional train (NS) (understand the "slow") : 2h45 for around 25-29 EUR each way if booked a few days in advance. The fastest one with the Thalys is 1h55 for around 90EUR. Driving take around the same time than the slowest way ~2h30.

The Amtrak trains are slow? I mean even adjusting for the potential 40 to 10km difference and the fact that the NS does between 8 to 10 stop depending where you want to go out in Brussels or Amsterdam, 4h for a 241km trip is slow.

It is a bit cheating as if you want to do Brussels - Arlon, a 190-ish km trip inside Belgium, it will take you 2h45. And if you want to more or less cross Belgium from North to South (Oostende to Arlon), 310-ish km trip will take you around 4h15 by train and between 3 to 4h by car due to the fact that you will take the Brussels ring road. So small country, yep. Still 4h for 241km is slow.