A video file being created on the hard disk while Dolphin-Preview active is being scanned for a thumbnail, every second although the creation is a minute or hour long process resulting in thousands of thumbnail scans on one file. The used up CPU and IO resources for scanning, extend the actual creation length of the file as less resources are available, resulting in even more thumbnail scans, as the overall process takes longer.


Reproducible: Always

Steps to Reproduce:
1.  create a huge video file, for instance recode video a to video b. Let resulting file be a modern hd file and let it have 5 to 10 gigabytes of data ( maybe smaller files also effected)
2. Have dolphin open, showing that folder. Preview Enabled.
3. As video b is constantly "growing" as it is being generated, dolphin or better the thumbnail.so re-scans the file every second, resulting in huge CPU usage.
4. Let the recode continue as long as it takes( this can be hours, depending), dolphin will try to generate new thumbnails for hours, in bad situations.


Actual Results:  
high cpu usage, desktop lock up/not reacting for seconds , every few seconds

Expected Results:  
nothing, until read/write operation finished on the file

installed thumbnail software version:

local/ffmpegthumbs 15.12.0-1
    FFmpeg-based thumbnail creator for video files
local/ffmpegthumbs-git r91.c06602f-1
    FFmpeg-based thumbnail creator for video files
local/kdegraphics-thumbnailers 15.12.0-1
    Thumbnailers for various graphics file formats