It is slow. And it is ok. Very few times will R ever beat any other language. Usually it is not off by much, but especially if coded by a novice using for loops vs apply functions can make is 100 -1000 x slower.
Another example is the immutable structure that causes R to be a memory hog. Creating copies of data everywhere. But, again if you plan well and execute the 'best' solutions you can avoid the giant pitfalls but will rarely ever beat a equally well written python equivalent.
Post R 3.1 there are far fewer deep copies (e.g. modifying a list or adding a column to a data.frame no longer copies the whole thing like it used to).
Another example is the immutable structure that causes R to be a memory hog. Creating copies of data everywhere. But, again if you plan well and execute the 'best' solutions you can avoid the giant pitfalls but will rarely ever beat a equally well written python equivalent.