Am I asking too much of my MacBooks performance? Will El Capitan help?
For now, try "Reduce transparency" in in System Preferences → Accessibility → Display.

El Capitan brings improvements to System Performance: 


  OS X El Capitan improves system performance across your Mac, making
  many of the things you do everyday faster and more responsive. Metal,
  Apple’s groundbreaking graphics technology, accelerates Core Animation
  and Core Graphics to boost system-level rendering by up to 50
  percent,* and efficiency by up to 40 percent,** resulting in faster
  graphics performance for everyday apps. Metal also takes full
  advantage of your CPU and GPU, delivering up to 10 times faster draw
  call performance for a richer, more fluid experience in games and pro
  apps.


Apple announced that Metal support in OS X extends to all Macs built since 2012:


  MacBook 12-inch (Early 2015)
  MacBook Air 11-inch (Mid 2012 and later)
  MacBook Air 13-inch (Mid 2012 and later)
  MacBook Pro 13-inch (Mid 2012)
  MacBook Pro 13-inch Retina (Late 2012 and later)
  MacBook Pro 15-inch (Mid 2012)
  MacBook Pro 15-inch Retina (Mid 2012 and later)
  Mac mini (late 2012 and later)
  iMac 21.5-inch (Late 2012 and later)
  iMac 27-inch (late 2012 and later)
  iMac Retina 5K, 27-inch (late 2013 and later)
  Mac Pro (Mid 2012)
  Mac Pro (Late 2013)


If reduce transparency doesn't help today, I say wait for El Cap and try again.