There have been complaints about Parallels (or VMware Fusion) slowing down Mac computers (for instance at Proz).
It would be interesting to investigate what can be the cause of this.
To install and set up Parallels Desktop 10 for Mac, you need:
A Mac computer with an Intel Core 2 Duo, Core i3, Core i5, Core i7, Core M or Xeon processor (Core Solo and Core Duo processors are no longer supported)
Minimum 2 GB of memory (4 GB of memory is recommended)
About 850 MB of disk space on the boot volume (Macintosh HD) for Parallels Desktop installation.
About 15 GB of disk space for each virtual machine.
SSD (Flash Storage) is recommended for greater performance.
The minimum system requirements for installing and using VMware Fusion 7 are:
Any 64--bit capable Intel® Mac® (Compatible with Core 2 Duo, Xeon, i3, i5, i7 processors or better)
Minimum 4GB of RAM
750MB free disk space for VMware Fusion and at least 5GB for each virtual machine
Mac OS X 10.8.0 or later
I have used Parallels for many years, without any performance problems. Recently I have switched to Fusion, because I find that it supports my iMac Retina better. (Very well possible that future versions of Parallels will provide good support for an iMac Retina too.) My Macs have an SSD.
Booting and quitting my VMs only takes a couple of seconds. I think that it is important not to assign too much RAM to your VM. Currently I've assigned 2 GB of RAM to my 64-bit Windows 8.1.
Hm. As long as Parallels does not run, it should not do anything evil (though there might be smaller services running all the time, but they shouldn't). If I remember correctly, Parallels in its newer versions tends to install software on the Mac side (antivirus and Parallels Access). I would not do this.
I've been using Parallels for many years, and I've never found it unsatisfactory so far.
But it seems that the retina resolution is not applied to some (maybe old) Windows applications, which still look ugly.
How about Fusion? I tried it once, but did not to all corners of it.
I'm using MacPro 2013 with a third-party 4K monitor.