Is there any alternate method for nmap UDP scan?

First of all you can use the -T5 parameter on nmap UDP scan, which is the fastest rate possible. By default, nmap uses -T4. Still you won't get as quick results as you would with masscan.

According to the creator's github: This is an Internet-scale port scanner. It can scan the entire Internet in under 6 minutes, transmitting 10 million packets per second, from a single machine. It's input/output is similar to nmap, the most famous port scanner. When in doubt, try one of those features. Internally, it uses asynchronous tranmissions, similar to port scanners like scanrand, unicornscan, and ZMap. It's more flexible, allowing arbitrary port and address ranges. n practice many penetration testers, including myself, use it to quickly scan devices for open ports and then take the results to nmap for more information.For example, the command below will scan a host on all TCP (-p1-65535) and UDP (-pU:1-65535). In just a few minutes you will have all open ports and you can use nmap to scan ONLY them for services etc. masscan -p1-65535,U:1-65535 10.10.10.x --rate=1000