laser,
Most of compilers, in general C compilers, give you option to generate an assembly listing, interlaced or not with your high level code, so that you can easily (.....relatively easier) check for execution time and decide how optimize your code.
However BASCOM AVR, does not gives for this facility, that is it generate .hex file except for assembly listing. This is true till the version 1.11.6.2 at least. Cannot guarantee if newer versions, if any, have this facility.
For criticals routine (i.e. I/O) you can use inline asm directive, like this:
$ASM
Ldi R24,1 'load register R24 with the constant 1
St X,R24 ;store 1 into var c
$END ASM
Or, if you have free pins you can set a pin when routine start and reset it when routine elapsed. You must have an oscilloscope.
Or, much better use a C compiler. Even though BASCOM is a compiled basic, claimed for optimized, it is still BASIC. So you cannot have full control how routine you wrote will optimized.
If you want stand at BASIC, you can try FASTAVR BASIC. Try out
www.fastavr.com. You should able to download a demo copy.
It generate for asm listing.