Note: It is also possible that the directory which you mentioned above is not on the same file system as /opt/ . It can be a symbolic link pointing to a different file system. Please recheck.
If the find takes a long time just to show files, my guess is that you directory is big.
There may be a race condition as well. Some file looks like they disappear between the time they're detected by find and deleted by rm (so the warning).
How big is a req file on average. Maybe you cannot expect your operation to free much space.