Say a Mac Pro tower has OS X + Apps installed on a blazing fast PCIe SSD. It also has Time Machine enabled on a slow 5400rpm mechanical drive. The TM drive is internal and connected at all times.
Will the relative slowness of the TM drive affect general performance when the Mac and its filesystem are being used strenuously? Are internally connected TM drives being being read/written to constantly?
Best Answer
In general, I would think that a slow Time Machine disk is not going to really affect the performance of your machine.
If you are using the 5400 RPM disk only for Time Machine, then the performance of that disk is only going to affect the speed that backups are made and recovered for the most part. It is true that when TM is running it is scanning your hard disk to determine what has changed, but you indicated that it is a PCIe connected SSD and thus I'd take a guess that you're probably not saturating that drive's performance most of the time. It does seem that TM runs at a lower priority and thus normal activity on the SSD is going to get precedence.
In general, the small slices that TM is going to steal from your SSD most likely aren't noticable and the slower performance of the 5400 RPM disk is isolated only to TM since you're not using it for anything else.
For some perspective, I have a 2008 MacPro3,1 with 1 SSD, 1 7200 and 2 5400 "green" drives in it and Time Machine goes out to a fairly slow ReadyNAS Duo on the network. When TM kicks off I can hear the drives in the MacPro chattering away, but I don't notice any slow down with what I'm doing since TM appears to be lower priority. However, attempting to go into TM to restore anything and it can be fairly abysmal. Fortunately, I rarely restore from backup :-)