Reindexer incredibly slow

When i reindex, the reindex is at 17% after 11 minutes.
When i check back after 37 minutes it was at 37%.
Now it is at 40% after 60 minutes.

The whole reindex process takes like nearly 4 hours at the end, which is awkward.
Why is it so slow?
I have like 150k products (in 2 languages, so every 75k).

Even in smaller shops with just a few thousand (~5500) products the indexer takes 45minutes.

This would be acceptable if it would run in the background.
But as long as it reindexes, the search is slow.
The indexer and the much faster search was the reason why i bought this plugin.

So then i will have 4hours long a shop with a slow search.

Do you have any tips or tricks?
Is there something i can do to speed this up?