I inserted a summary to my slides which is very long.
It inserted everything to one page. I think it should be better if it was in many pages.
However, let’s try to solve the symptom, not the main problem.
How can you format the content to be multicolumn?
If you use Impress, just make two text frames and fill these with your text.
If you happen to use Writer for this, then you can do this:
You could have page with multiple columns. Since you have other pages as well, let’s create a new page style first (in styles and formatting dialog, F11). Set it to have 2 columns. If you are done, place your cursor right before the summary and from menu Insert → Manual break → Page break and use your newly created style. After the summary use similar break with Default style. (BTW, the same goes to having landscape pages as well)
If you need the summary (or whatever two column area) to start in the middle of the page (page break is not possible), use panels instead. See Insert → Panel → Columns.
@Masi - @oweng’s answer is correct. However, if your document is a real presentation and not only a document in Impress format, I would recommend to reconsider if the summary slide is not too long to keep the attention of the audience.
You have done it again 
@mahfiaz - it seems I have a bug in a brain! Sorry nor not crediting you!
@oweng - I’ll do my best to solve the bug!!!
@ROSt53, no problem Reading your comment just made me realise that he might use Impress not Writer (he mentions slides). So I edited my answer accordingly.