Don't upgrade until you've had a jolly good troubleshoot. The 2015 model will only be marginally faster than a correctly working 2014 model.
NB: That
doesn't mean install any of those iffy 'mac cleaner' applications you see in the pop-up ads. In fact, if you've got one of those it might be part of the problem...
One obvious thing to try before doing a clean OS X install (as suggested above) - is the hard drive full? You want at least 10% free space or things could slow down.
(NB: I don't know the following applies to Apple's SSDs - this is clearly for Crucial drives - but it won't do any harm:
http://forum.crucial.com/t5/tkb/articleprintpage/tkb-id/ssd@tkb/article-id/84)