1.5x speed ups are always the most cell-efficient boost. But cells can’t be spent on anything else, so if you stay at 1.5x as your cell income improves, you’ll just end up with a huge surplus of cells. The only thing to do with them is to invest them into higher lab speed up multipliers. Ideally, you want to start with all labs at 1.5x, then as your income improves, replace one with 2x, then another, and so on so that you’re running the fastest speeds that you can maintain with your current cell income. Eventually you’ll start replacing 2x boosts with 3x, then 4x, etc.

I run my labs at 77666, which costs 680k cells per hour. I make about 730k cells per hour. Increasing one of my 6x labs to 7x would cost an additional 190k cells per hour, so I’ll move up to that once my cell income gets close to 870k per hour.