[DataFrame] iterrows() is slow for larger dataframes

Runtime on `iterrows()` on larger dataframes is slow:
```python
df = rdf.DataFrame({'A': list(range(100000))})
%time df.iterrows()
```
Output:
```
CPU times: user 47.1 s, sys: 617 ms, total: 47.8 s
Wall time: 1min 13s
```