Is it fine if I Dual Boot Windows 8.1 And Ubuntu. My hard disk space is 500gbonly. Will the performance of my laptop decrease??
Yes, it is fine. No, performance of your laptop will not decrease, unless you will make weird partioning by yourself.
Personally, I prefer to give Windows as much space as possible, since it has a bigger appetite for resources. So, on your place, I would give Ubuntu 30Gb for root partition + 20Gb for home partition, and the rest allocate to Windows. If 20Gb is not enough, you can always mount a Windows partition(s) and keep your files in one place.
Yes it is.. you need to be careful during the portioning . This link may help you dual-boot-windows-8-1-and-ubuntu