What is easier or faster depends a lot on the specific use case. 
Separate tags makes parsing the value slightly quicker (you save just a simple split operation of a string), but on the other hand it makes finding the tags a lot more expensive: Your database request doesn't ask for a simple key, but you need a wildcard search for numbers as subkeys.

In general, I prefer the list in a single tag -  any software has to be able to cope with this syntax, supporting yet another tag is a non-issue.