If you rely on SOFTWARE to do your Comms reception, then you will have to rely on the timeout values built-in to the software... if you use the PICs USARTs then you can play with the data a microsecond (or less) after the PIC has caught it - and I don't mean using HSERIN as an alternative either! Go access those registers directly (full step-by-step instructions in the Datasheet).

There's so much rubbish built-in to the PICBasic serial routines to give you a shed-load of features (that most folks will never use but everyone wants) and make it easy to use (because people struggle handling 'byte-at-a-time'), that something has to give... and that something is speed.

If you don't believe me how bloated the comms software is... see by how many words your program grows when you try to catch ONE BYTE using SERIN, SERIN2, DEBUGIN or HSERIN. By accessing the PICs comms hardware (assuming it's a PIC that has a USART), you can do it in less than ten instructions (that's including setup).