Going by the rule of thumb "the more data on your hard drive the slower your computer will be" I always thought when you partition a hard drive into an OS X disk and a Windows 7 disk via Boot Camp, the computer speed will be unaffected because when one operating system is running it doesn't have access to the other operating system's disk.
But I remembered that the operating systems do have access to each other's disk space (e.g. if you're in the mac side you can use access the Windows disk and retrieve data).
Does adding Windows via Boot Camp slow down OS X because it has to handle all of the Windows data?
Best Answer
As I understand it, what you what to do via "Boot Camp" is to reduce the size of one or more existing partition(s) and then creating a new partition in the (now vacant) space.
While the resizing of the original partitions takes place, there will be serious performance degradation, as any data stored in the space to be freed (freeed?) must be read from and written back to the disk. This will however only last for a endurable amount of time.
After you have installed another operating system, there are several potential (but small) ways in which performance could be degraded:
If you have some kind of indexing application (sorry, I'm not too familiar with Macos) that can and is configured to access the new partition, it will cause addional reads every time it scans the partition. It should be easy to disable this, though, if it happens at all.
The partitions you use are now smaller than they used to. Most filesystems under most circumstances will show very poor performance if there is very litte space left on it. How much is very little? That depends, but conventional file systems (including hfs+ afaik) usually work splendidly at least up to 90% utilisation.
The process of reducing the size of the filesystem could (in fact this is very likely) cause a large increase in file system fragmentation that will reduce the read speed, particularly for large files. Again, how large this issue is depends on hoe much free space you have available, but if your result is less than 50% free space, I would suggest finding some method of defragmenting the file system. Apple doesn't want to solve this problem but apparently there are third-party tools available.