Your card seems sluggish because it is.
Your particular PNY SD card is rated at up to 60MB/s (megabytes per second) data transfer rate; which is approximately .5Gb/s (gigabits per second)
In comparison, your SanDisk has a read speed of 150MB/s or 1.2Gb/s (SanDisk does not provide a write speed).
Basically, you swapped out a storage medium for another that, out of the gate, is at best less than 1/2 the speed than what you had originallyBasically, you swapped out a storage medium for another that, out of the gate is at best, less than 1/2 the speed than what you had originally. There is no filesystem that is going to make up that difference.
For reference USB 3.0 is rated at 5Gb/s which is equivalent to 625MB/s.
Format Types
"Using it with DropBox" doesn't have any effect on the file system type you choose to use as DropBox's functionality and performance has nothing to do with where your OS writes its files to. So, the question becomes, which one is is right for you?
- Use ExFAT if you plan to go between OSes like Windows and Linux
- Use HFS+ if you plan on only working on OS X and like all the extras you get with HFS+
If you really want (need) Speed
Get an external drive that takes full advantage of USB 3 speeds (up to 5Gb/s) like the WD Passport for Mac or go for a Thunderbolt Drive that will go 10Gb/s