One other feature of FL Studio that will help with your problem is the Smart Disable Macro, which automatically shuts off any plugins when they aren't processing audio. Go to the top-left menu and hit Tools > Macros > Switch smart disable for all plugins.
Doing this gives back a huge chunk of CPU in large projects, but it does have a couple of drawbacks: you will sometimes have trouble with tempo-synced plugins or ones that draw a small latency, such as Xfer's LFO Tool, or look-ahead compressors and limiters; and you will still have the computer choke when playing back big sections with a lot going on. It will all return to normal upon rendering, though, so don't stress.
Using a combination of rendering parts down, and the Smart Disable macro, makes life a little easier with heavy FL projects. Another tip you could try is a process-killing utility designed for gaming. Sounds strange, but Razer's GameBooster works just as well with FL Studio as it does with any game - just set it to launch with FL and it'll free up a lot of processing power for your music.