What is the maximum value for a drop down list?

You mean the number of choices? I do not know of a concrete limit. But performance and UX will degrade when that number becomes 3 digit or more.

I agree with Stefan that if you have more than about 100, you should consider using other options, such as cascading dropdowns or pickers. The only exception to that is if you have something where there are a large number of familiar values - for instance, I'd rather have a dropdown for all countries, even though there are ~200 of them.

It's hard to answer what the performance impact will be except to say, the more options you have the slower it will get. It is highly dependent on what your data source is, how you're querying / manipulating the data, etc.

I agree with Stefan that if you have more than about 100, you should consider using other options, such as cascading dropdowns or pickers. The only exception to that is if you have something where there are a large number of familiar values - for instance, I'd rather have a dropdown for all countries, even though there are ~200 of them.

It's hard to answer what the performance impact will be except to say, the more options you have the slower it will get. It is highly dependent on what your data source is, how you're querying / manipulating the data, etc.