Linux and Windows combined in 1 PC? Safe?

It is very easy and safe to use two operating systems on a same PC. Linux is a great operating system and you have to spend nothing on it as it is open source. The problem of not detecting Wifi fast can be due to other reasons not because of installing linux on the pc. Try to boost up your pc by removing unwanted data, disk defragmenter, registry cleaner and other tools readily available for windows platform and your problem will be solved and a better response time will be achieved.

Running both OSes (Windows & Linux) is absolutely safe as long as the boot file is working fine (you can correctly boot into both the operating systems). It is advised to install Linux after installing windows because Windows doesn't detect the presence of another (Non-windows) operating system thus creating conflicts in the boot file. On the other hand, Linux does consider all other Operating systems (already installed on your PC) and creates a boot file of its own (the GRUB) enlisting all other OSes which where already installed in the PC. About the issue of slow Wifi detection, I don't think it has anything to do with having two Operating systems, should be solely related to the connection you use and the parameters related to it.

It is more a Windows problem than a Linux problem : Windows needs optimization and defragmentation to keep its performances. Linux use its own partitions, at least one main partition called "root" and sometimes a cache partition called "swap". The format of Linux partitions cannot be natively read by Windows, so they cannot interact under Windows. There are quite no malware under Linux, and a Linux partition can be very useful to check Windows partitions for malware and virus : no anti-virus is completely secure.

Linux is the safer system and should be used for connecting to the Internet as a matter of security. Windows has many bad habits, one of which is that it gets slower and slower over time - as it grows in size! The Wifi thing has nothing to do with Linux whatsoever. It could be the result of malware or virus on your computer. If you use Linux instead, you wont have to worry about such nasty things.