The biggest reform was in 1911 when the Parliament Act abolished the Lord's power to veto legislation and replaced it with a delay of up to three years. The answer to your question depends on the period and how you judge a legislature. As late as the 1890s it was possible for the Prime Minister to sit in the Lords (Salisbury) so it was a reasonably effective chamber. The problem occurred when the Liberal government of 1905-10 had a large Commons majority but was a minority in the Lords. This culminated in the Lords vetoing the 1909 budget on the grounds that it was a huge social reform programme in disguise. The constitutional crisis which followed led to the loss of most the Lord's power. I know that period best but my understanding is that the Lords was far more important in the early 1800s. In the 20th century the Lords was more of a nuisance for Labour. It was less powerful than it appeared as few hereditary peers were active in politics. It also usually played by unwritten rules that a government could pass measures that were election commitments. A big reason why change was so slow was that any elected upper house would have the legitimacy to challenge the Commons and governments don't want that.