I was planning to go to universal studios within the Christmas week I can only find regular ticket - no fast pass. will it be too crowded so that it'd be annoying ?
It will absolutely be crowded Christmas week as it is one of the busiest weeks of the year if not the busiest week of the year. According to IsItPacked.com, crowd levels range from "Yup, It's Packed" (40-60 minute average wait times) to "Forget About It" (60-90 minute average wait times).
There is no fast pass system at Universal Studios. However, instead of purchasing a General Admission ticket, you can purchase a ticket called "Universal Express" that grants you express access to each ride a single time. This costs between $179-$279 depending on the date (expect to pay in the upper range for Christmas week).