Will a USB 2.0 5400RPM External HDD for Time Machine slow down my SSD?
I use a USB 2.0 external hard drive for Time Machine, and I have not seen any changes in speed.  Time Machine works pretty quietly and effortlessly in the background without slowing down the Mac's performance.  According to the Time Machine System Preferences pane, 


  Time Machine keeps:
  
  
  Hourly backups for the past 24 hours
  Daily backups for the past month
  Weekly backups for all previous months
  
  
  The oldest backups are deleted when your disk becomes full.