Tables taking too long to merge in Power Query

As first level of performance improvement, do following -

1. Remove all columns which are not required for your analysis / decision and not required for merging in botht the tables.

2. Also if you can apply filter to minimize number of rows required to do merging.

3. File - Options & settings - Options - Current File - Data load -Uncheck "Allow data previews to download in the background"

4. You can also explore possibility of using Table.Join as given here - https://blog.crossjoin.co.uk/2020/06/07/optimising-the-performance-of-power-query-merges-in-power-bi-part-3-table-join-and-sortmerge/