If you got a DDR4-supporting laptop, or perhaps a mITX system that requires DDR4 SODIMMs with 2666 MHz support out of the box, you don't have that many options today. Team Group's T-Force Vulcan Gaming Series of DDR4 SODIMMs come ready for that and more.
Slow computer got you down? Maybe a hardware upgrade should be in your cards, if so this guide to the Fundamentals of Upgrading a PC is an invaluable read. PCSTATS Tips