Decrease Insert and Update performance on bigger database
Assuming you're using Microsoft SQL Server (as your tag implies), if you're using JMeter, ensure your tests run directly on SQL Server and don't involve any middleman that can affect results. I'm not personally familiar with JMeter but the biggest difference you have between your two tests is about 57 milliseconds which is little enough to be affected by any possible variable in your process.
Although the difference between your two InsertNewTransaction tests is about a 35% difference, it's still only 57 milliseconds, which is very small. It can even be just the difference of a single run of one test vs it's second run, or more outside factors like temporary hardware blips. You'd have to run a large number of instances of each test to average and really compare when you're dealing with such granular time measurements.
That being said, some general advise is as follows:

INSERTS take longer when there are more indexes on a Table. So when applicable, reducing the number of low-use or unused indexes can help your INSERT performance.

Partitioning isn't meant to improve performance from a SELECT or INSERT perspective. Rather it helps with better granularity on maintaining the data, and can be used smartly to improve maintenance tasks performance (such as index maintenance).

If the table with 56 columns is being slow, then normalizing it, if possible, can help your INSERT performance (especially if you're not always inserting data for all 56 fields) and can help your SELECT performance.

If you need further performance critical changes, you can look into if Memory-Optimized Tables fit your use cases.