Better to Balance Research Or Avoid Focus?

[Sandermatt]

For long term research progress, should I from time to time focus technologies that are behind time, or should I avoid focuses at all? DO we know the mechanics of research speed?

[madham82]

I typically only focus when you actually get the turn counter on something I want. It seems keeping focus on slows down progression on all other techs otherwise. Interested to hear others as the mechanics are not explained.