Do you have on-demand (read) AV running?
Whenever a large file is opened by an application, the AV scanner is going to scan it - and in the case of videos the files are going to be v. large indeed.
When AV is running it will usually take up nigh-on 100% of CPU time, so everything else - including TM - just has to wait.
If you do a normal AV scan on the video file(s), you could then add them to the exclusions list for the AV scanner.
I found that java
was causing this on my system, so I added the jave .exe's to the exclusions list and it improved the situation considerably.
As a matter of interest - AMD or intel?
SD