Each tier of vanilla lab research speed unlocks three (or four) additional technologies which "pay back" a portion of the newly acquired speed bonus to gain +1% laboratory productivity instead; if all lab speed and efficiency technologies are researched, the net result is +70% speed and +20% productivity (instead of +250% speed). Of course, unless you're a completionist and it bugs you to leave things unresearched, you can stop at any point you find comfortable if you'd rather have the greater speed.