It's just that some people have very short memory and knee-jerking to reach hasty conclusion, as usual.
I always remember Mercedes is collecting huge mileage in the middle of timesheet in winter testings and I thought it was common understanding. For example aggregated time for 2016 first test is below. Some other years they were up there but generally Merc tend to stick to data gathering runs or lots of long runs with mediocre time in testing.
What's different for this year is reg alteration mainly for front wing, "merc wing is different - they screwed!" voila, easy.
https://www.formula1.com/en/latest/feat ... hest-.html
POS DRIVER TEAM/CAR BEST TIME GAP LAPS
1 Sebastian Vettel Ferrari SF16-H 1:22.810 195
2 Nico Hulkenberg Force India-Mercedes VJM09 1:23.110 0.300 99
3 Kimi Raikkonen Ferrari SF16-H 1:23.477 0.667 158
4 Daniel Ricciardo Red Bull Racing-TAG Heuer RB12 1:23.525 0.725 199
5 Sergio Perez Force India-Mercedes VJM09 1:23.650 0.840 101
6 Daniil Kvyat Red Bull Racing-TAG Heuer RB12 1:24.293 1.483 170
7 Alfonso Celis Force India-Mercedes VJM09 1:24.840 2.030 133
8 Nico Rosberg Mercedes F1 W07 Hybrid 1:24.867 2.057 332
9 Marcus Ericsson Sauber-Ferrari C34 1:25.237 2.427 196
10 Kevin Magnussen Renault R.S.16 1:25.263 2.453 264
11 Max Verstappen Toro Rosso-Ferrari STR11 1:25.393 2.583 231
12 Lewis Hamilton Mercedes F1 W07 Hybrid 1:25.409 2.599 343
13 Esteban Gutierrez Haas-Ferrari VF-16 1:25.524 2.714 168
14 Valtteri Bottas Williams-Mercedes FW38 1:25.648 2.838 214
15 Romain Grosjean Haas-Ferrari VF-16 1:25.874 3.064 113
16 Pascal Wehrlein Manor Racing-Mercedes MRT05 1:25.925 3.115 125
17 Felipe Nasr Sauber-Ferrari C34 1:26.053 3.243 236
18 Fernando Alonso McLaren-Honda MP4-31 1:26.082 3.272 122
19 Jolyon Palmer Renault R.S.16 1:26.189 3.379 79
20 Carlos Sainz Toro Rosso-Ferrari STR11 1:26.239 3.429 216
21 Felipe Massa Williams-Mercedes FW38 1:26.483 3.673 163
22 Jenson Button McLaren-Honda MP4-31 1:26.735 3.925 135
23 Rio Haryanto Manor Racing-Mercedes MRT05 1:28.249 5.439 129