This is a common White House refrain that has been extensively discussed and fact-checked. See here, here, and here, and see also an earlier claim with more specific historical references.
Yet as even The Washington Post has reported, the Senate is conducting the slowest confirmation process in American history.
But it is historic in the level of obstruction that is denying the will of the American people who elected a new administration and expect that administration to be able to staff the departments.
—White House Director of Legislative Affairs Marc Short
Press Briefing, July 10, 2017
The Washington Post has checked this statement.
This is our first Cabinet meeting with the entire Cabinet present. The confirmation process has been record-setting long — and I mean record-setting long — with some of the finest people in our country being delayed and delayed and delayed.