Logic indexing optimization of computational time

I am running a computationally expensive code, and according to MATLAB's profiler, this is the line of code that is taking the most time to run:

b   = a(idx,:);

where a is 30000x3 matrix, and idx is a logical 30000x1 array, computed as follows:

idx = a(:,3) >= M + N;

where M and N are integers.

That line of code is run inside a for loop with many other lines with functions such as find. I need to understand why that line of code is taking longer to run than all other lines, and how I could optimize it. Sorry for the low amount of information on the code, for property reasons I cannot divulge much more.

Thank you in advance!

[Torsten]

Disregarding the advice to compute M + N in advance, nothing can be said because of the little information you are willing to supply:

MN = M + N;
idx = a(:,3) >= MN;

[Rafael Félix Soriano]

Thank you Torsten. Sorry, I know is little (to nothing) to go on, but it is not up to me to provide this little information about the code.

[Dyuman Joshi]

find() could be a trouble maker here, but as Torsten mentioned, we can't provide a definite answer to your question without more information.

You can't share a snippet of your code as well (just that particular line) ?

[Rafael Félix Soriano]

Thank you for your answer Dyuman. My point is that having several lines of code running find, none of them are taking long. It is the one that I posted (b = a(idx,:)) that is taking significantly longer than all others. That is the only part of the code giving me problems.

I am sorry but I am not allowed to divulge any more info.

[Torsten]

Often, it is the interaction of many factors that makes a code slow. So concentrating on a single command is not the correct approach in my opinion.

That having said, b = a(idx,:) is the optimal way to extract the rows of the matrix "a" where the element of the 3rd column is >= M+N. Nothing to optimize or critisize here.