If the audio files are recorded at 32-bit, converting them down to 24-bit can help relieve stress on the audio drive. To do this:What do you think, guys, could I optimize something else or the only way to get over this is a faster hard drive?
1) Go to Preferences > Record, Warp, Launch and change Bit Depth to 24
2) Open the project and create a copy using Save As
3) Select All audio clips, Right-Click on a clip and select Crop Clips
This last step can also be done individually per clip, rather than by selecting all. When in the Detail View or right-clicking on a single clip, the function changes to Crop Sample. When the Crop function is used without selecting a region of the clip, it creates a new copy of the entire clip; and since the Preferences were changed, the new clips will have a 24-bit word length, which should be easier on the audio drive.
Worth a try, hope it helps. If you are already running 24-bit, consider converting to 16-bit as a test.
PS. One thing to note when installing a 7200 rpm drive in a Macbook is that the heat will increase which will cause the fan to turn on more frequently. We have a 7200 rpm drive in my wife's MacBook, and when doing any audio or video work for more than about 10 minutes, the fan stays on pretty much all the time. It's just a compromise to consider when weighing the pros and cons of an upgrade.