This happens on any map, at any distance, and with any ammount of explosions. Seems like map optimization won't affect the explosion effect, even if you're in the jail of Bunker, you will still get fps drops from explosions occurring on the main room. Or in Facility Backzone while you're in the red room with armor you will get fps drops from explosions on the bottling room.
(http://i.imgur.com/1CrohX2.jpg)
(http://i.imgur.com/GfMXlJw.jpg)
(http://i.imgur.com/dyChEnq.jpg)
The first one is before any explosions, the second one during one explosion on the main room, and the last one is one explosion in front of me. I never got less than 30fps with explosions, the fps are 30-100, with all the effects disabled, 60-100. This is really noticeable and annoying while playing, in aiming mainly. If I limit the fps to 150 (I normally have them limited at that), the fps drops are the same.
Here my PC specs (Playing at max settings, 1080p):
GPU - R9 280x 3GBvRAM
CPU - i7 3770 @ 3.4GHz with a max of 4.4GHz. 8 cores.
RAM - 8GB 1600MHz