your GPU drivers need updating, last update was 09/28/2016.
Since it is a U class CPU you may be locked to dell drivers - please go to dell and update to the latest version they have. Then try updating from the intel driver site, for the latest version.
This could be combination of a couple of things:
1) Driver outdated
2) Blockage on the heatsink from dust and junk - even small amount could cause the FPS drops - which from experience this sounds like.
2) Driver and dust blockage on the heatsink
How to resolve the heat-sink, blockage is to get a can of compressed air and gently blow the heatsink and fans until dust has gone. - please seek guidance, on how to do this from your dell manual.
Intel driver tools - http://www.intel.com/content/www/us/en/support/detect.html
Dell - please go to dell.com and then service area and enter your service tag into tool to find your laptop o7