One aspect that must be factored in, however, is the added weight from the now-mandatory Halo safety device. This equipment has been a hassle for teams as they develop their cars for next season and the additional heft could slow down times according to Isola.
"If we go softer with the tires, the cars could be ever quicker. But we have to remember that there is an additional weight with the Halo, and that will cost about three to four-tenths of a second, because of the weight plus aero implications."
The FIA bumped the minimum weight of the Halo to 6 kilograms for 2018, but teams like Force India have found that the necessary hardware and chassis bracing actually increases the load even more. This, along with the complicated structural integrity of the equipment, could make R&D even more difficult leading up to the season-opening Australian Grand Prix.
Source : http://www.thedrive.com/accelerator/17028/formula-1-cars-projected-to-be-two-seconds-quicker-in-2018