does merging tables always increases performance? Consider we have data from visitors in a web site. And you are trying to create a database design for data mining.

There are some common fields which users have such as browser,ip, etc... . But there is a field such as email or something which for example 20% of people give. 
My question is at what percent you should merge tables and where we should move the email or custom field to another table?

in this case, is database size the only drawback of merging tables? and is it's performance results better than seperating table?
Without any knowledge about the workload of the database, is almost impossible to answer to a question like this. How often those “extra” data will be used? What are the most common queries? How frequent they are? and how much it is important the speed of the execution? 

The general advice is always the best one for me: make things as simple as possible, than optimize if you need to.

So my advice is: create a single table, look at the performances, and only if those performances are not adequate to your objectives, split the table.