Optimising rendering when using HDRI's and emissive lighting.

Superdog

I find that using HDRI's slows down renders compared to the daylight setting. Also the viewport takes longer to update when I use HDRI's. A scene where I used spotlights took much longer to render. What I'd like to discuss is how to reduce render times when using HDRI's and emissive lighting. Is it possible to reduce the resolution of emissive lighting while still maintaining the lighting effect and reduce render times? What type of HDRI's use less resources and how can I adjust them to render faster? Any help/advice appreciated.