Do I need to upgrade my late 2011 Macbook Pro I currently own a MacBook Pro, Late 2011, With a 2.4ghz intel core i5, 4gb memory DD3. I use my mac mostly for web browsing and iTunes, and use (bootcamp) windows for programming. Recently, I have seen my mac slow down at times. When I click an application on my dock, takes aprox. 5-10 secs to open the application. I have seen lag when having multiple applications open at once. Running Windows 8, I do not see any lag unless I am running the debugger on visual studio but that could a VS issue. One of the reasons I want to upgrade is booting time. I am constantly changing going from OSX to Windows and can take up to 30-45 seconds to boot into either operating systems.

My question is asking for your opinion of what Should I do to improve performance. Do I upgrade my current MacBook as it is still In good condition and runs relatively fine. Or do I buy a new Mac with a better processor, more ram?
TL;DR: upgrade your laptop with at least 8 GB RAM (2 x 4 GB) and a quality SSD.
I have the exact same 13&quot; MacBook, same original specs and everything.  I got it used and immediately upgraded to 8GB RAM and a 128 GB Samsung 840 Pro SSD. It absolutely flies now!
I use various IDEs, Logic Pro for making music, VLC to watch videos, lots of MS Word and Excel, and sometimes have ~20 tabs open in Chromium, never have any problem with speed or system freezes. It runs like a clock, whatever I throw at it.
The cost of the upgrade was about $240 (at the time) and it gave this computer a new life. Absolutely worth it. Note:  I used an optical bay adapter to install a second hard drive and have the SSD paired with a 500 GB 7200 RPM Seagate HDD.
For your particular usage profile, the only thing you would likely notice in going from an upgraded 2011 MBP to a 2015 MBP of same specs is an increase in battery life.  Probably not worth the extra $1000, since you didn't list battery life as a problem.