I may be able to explain what you are perceiving as "slower."
The cast time for Sniper Blast has not changed, it still is 1.33 seconds. However, if executed in fast mode, it would just shoot at frame zero, then a few frames later it would execute the arm movement, and stay there, slowly waiting to lower the hand for about 1.33 seconds. Basically, the entire FX and animation were all out of sync.
The animation for all the 1.33 second snipes was altered so that now you lift your arm, point, and at frame 17 the shot is fired, then much quicker, but without skipping frames, the arm is lowered in place.
The game internally runs at 30 frames per second, so 17th frame would be 0.567 seconds into the attack. That may give you a sense that things slowed down, but it also means you now spend less time after the shot rooted doing nothing.
I understand that this may be a bit disorienting, but almost none of the interruptible sequencers were designed with the elimination of the interrupt window in mind. Only ones I think actually did OK were Blazing Bolt and Ranged Shot. Assault rifle was given a whole different animation in fast-mode, and this became way too apparent when the default animation at character creation was changed to display the fast version.
Rest assured this does not change the DPA of the fast snipes.