Battery drains very quickly on Samsung Galaxy S5
"....the battery suddenly fell from 100% to 95% in just 5 to 10 minutes"


That's perfectly normal and no cause for concern. Reasons for this are well explained in this XDA post  Your battery gauge is lying to you (and it's not such a bad thing). Summarizing from this post:


  As explained at Battery University, "The time at which the battery stays at [maximum charge] should be as short as possible. Prolonged high voltage promotes corrosion, especially at elevated temperatures."


so


  This is why many new phones will "lose" up to 10% within a few minutes of coming off the charger. The reality is that the battery was only at 100% capacity for a brief moment, after which the battery management system allowed it to slowly dip down to around 90%


so how do OEM's deal with this situation?


  The phone manufacturers essentially have three choices:
  
  
  Use older charging styles which actually maintain a full battery, thereby decreasing its eventual life
  Use new charging methods and have an accurate battery gauge
  Use new charging methods and have the inaccurate battery gauge
  
  
  Option one has clearly fallen out of favor as it prematurely wears devices. Option two, while being honest, would most likely be met with many complaints. After all, how many people want to see their phone draining down to 90% while it is still plugged in? Option three therefore offers an odd compromise. Maybe phone companies think that users will be less likely to worry about a quick drop off the charger than they will worry about a "defective" charger that doesn't keep their phone at 100% while plugged in.


So, it's normal and have seen it happen in every phone (four of them) I owned in last four years