Optimizing conference paper solution to be eligible for Journal article I have presented recently in one of IEEE computer science conferences. My paper's main contributions was optimizing an algorithm to achieve 3.5X times speed up. Now, I am planning to submit another paper to one of Springer's journals with IF 0.7. However, the only new contribution here is I optimized the gain to be 4.5X in addition to theoretical comparison between the algorithm and other 3 algorithms to justify why I have chosen this algorithm to be improved. The paper has been re-written completely and I am aware of the plagiarism issue. However, I am still concerned, is that contribution sufficient to make my paper qualified?
Let the editors and/or reviewers decide whether the new contribution is sufficient. Mention that you extend previous work. It may be wise to be explicit about the novelties with regards to the previous work in the cover letter.

In case the original paper was published in conference proceedings (e.g. not just a talk), it is very important that you cite it. When you do that, there can be no claim that you are attempting to recycle existing material without anyone noticing (self-plagiarism).