Marwan, please give us more information. We need to know what version of the Adobe software you are using, and the wrapper/codec of your video/audio clips. In addition, let us know if you recently upgraded to a new version.
In addition, tell us about your computer. What OS, how much RAM, your storage setup, etc. I keep all this information in my signature, so whenever I post, folks who respond have all the info.
System:
Dell Precision T7600 (x2)
Win 7 64-bit
32GB RAM
Adobe CC 2017.1 (as of 8/2017)
256GB SSD system drive
4 internal media drives RAID 5
Typically cutting short form from UHD MP4, HD MP4, and HD P2 MXF.