On your keyboard press CRTL + SHIFT + ESC or CRTL + ALT + DELETE. Something like this should pop up:
Right-click on 'Minecraft Launcher' and then click on 'Go To Process', like this:
'javaw.exe' should now be highlighted. Next, right click on 'javaw.exe' and move your mouse over 'Set Priority' and finally click 'High', like this:
After you've done that, your Minecraft should speed up a bit.