I've done a lot of testing on this.
Maximum germination rate and speediest germination occurs when the soil temp is 80 to 85 degrees. I set my thermostat for 82 now.
As temperatures go up from 85 or down from 80, the % germination will decline, and the time to germinate will increase.
Going down from 80 degrees, there is a dramatic drop off around 70-72 degrees, where germ rates and times fall off a cliff.
In order to get the typical 85-90 % germination and to see some green in the typical 5-8 days, you really have to get that soil temp to around 80. Which may require a heating mat.
Of course, once you see some green, the temp needs to be taken down - you need to turn the heating mat off. Ideally, after germination the soil temp would get into the low to mid 70s and kept there.