Many hardware reasons can prevent MIST or ICE to playback in realtime.
If the first playback only is difficult, and mostly for image sequences at a high frame rate, then the Windows antivirus may be the cause of the problem.
When MIST or ICE access a file for the first time, the antivirus is preventing the application to access the disk and read the file until the scan for virus is finished. In the case of image sequences, it will do that for every image. Therefore the current media drive must be excluded from the automatic checks.
To exclude some disks drives for automatic antivirus scannings you need to manage the settings in:
Windows Settings > Privacy & Security > Windows security
Open the Virus & Threat protection menu:
Click on Manage Settings:
Click on Add or Remove exclusions:
-> define the disk drives to be excluded from the scans.