Spoiler effects

[ElCaramba]

Hi,

I just wanted to get your opinion on the spoiler impact during descend.

During descend (-3500 ft/min, 35.000ft) I found myself slightly accelerating, so I decided to deploy 1/4 spoiler, just to find the airplane strongly decelerating. 
The Fenix decelerates a lot less compared to the A350 with spoilers deployed, so I wondered if the spoiler effects are somewhat too strong. Interestingly, 1/2, 3/4 and 1/1 spoilers don’t add that much deceleration (not linear).

Opinions?