First, why is it such a worry? Are you reading that spreadsheet in a loop? If so, why? Does it change each time? If not, why does it matter if the initial import takes a bit longer?
Note that unless you're using the 'basic' mode of xlsread or 'UseExcel', false of readtable, then matlab has first to start Excel before it can even read the data from your file. As you probably know, Excel can take a while to start and that's totally out of the control of matlab. Once excel has started, it's excel that opens the file. Again, out of the control of matlab. Finally, once all that is done, matlab can ask excel for the content of the spreadsheet and to be honest whether you ask for the whole lot or just 1 in 5 row is not going to make much difference compared to the rest. It may be that actually skipping rows may take longer since you need to spend time computing which cells need copying rather than copying the whole lot in one go.
If you use 'basic' mode or equivalent, then you're relying solely on matlab to parse the file. It's not as powerful but it may be faster since you don't have to wait around for excel to do its thing.
But to be honest, I don't think it's something you should worry about until you've proven that it's actually a problem. Premature optimisation is wasted effort. So have you actually timed how long it takes to read your data?
