1964Nurse....You're a new MDS Coordinator; you are going to be slower.
Quite honestly, it took me a good 2-3 years before I felt I could do assessments fast & a good 5 years before I felt like I could do them "real fast."
Hang in there; get all the training you can....The longer you're there, the more experience you will get & the more marketable you will be.
Also, the sloppier the charts are, the longer it takes to complete MDS Assessments.
And you have to consider ALL the daily interruptions....
Don't apologize to them; you have done nothing wrong....that Administrator probably does not have the first clue about the MDS Position (most don't).
If you like MDS work, just go to work with the mindset that it is one more day of experience that will help you land your next job.