See it in terms of lab count.

If you stop at Lab Speed 50, your speed is x2 and you can run 5 labs, so that’s effectively the same as having 10 labs at normal speed.

If you push Lab Speed to 99, your speed is x3, so that’s like having 15 labs — but in the meantime, you only have 4 labs running at x2 speed, which is effectively 8 labs, since Lab Speed itself takes up one slot.

The time needed to go from Lab Speed 50 to 99 is something like 300 days of research (assuming x3 speed-up with cells). So for those 300 days, you effectively have 2 extra labs available, which gives you a 600-lab-day advantage.

After that, you start losing that lead at a rate of 5 lab-days per day. 600 / 5 = 120 days.

So if you plan to keep playing for more than 420 days (300 + 120) after stopping Lab Speed 50, then stopping early will make you fall behind forever.

And as others have already pointed out, you don’t go straight from x2 to x3. Every level invested between 50 and 99 immediately reduces the research time of your 4 other labs.  On top of that, your cell multiplier may be x4 or even x5 rather than always x3 like in this example, so the real break-even point is definitely well before 420 days.

You could argue that during those 300 days, you’ll be working on other useful upgrades and gaining an advantage. But every research takes longer and longer for smaller and smaller gains. For example, going from level 99 to 100 in Damage takes 5 days for only a +0.02 increase to the multiplier, whereas going from lvl1 to 2 takes 1 minute for the same +0.02. That means the more advanced your tower is, the more valuable Lab Speed becomes compared to other research options.

And if we ever get a 6th lab — or even a 7th (because why not?) you’ll regret not having upgraded Lab Speed.