Get is between 10% and 30% slower from release 4.2 to 5.14.3

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Get is 10% to 30% slower. Maybe this is worth investigating.

### Expected behavior

10% slower over 2 years of releases seems OK to me. But 20% or 30% might be worth fixing.

### Actual behavior

Results are provided for (key,value) with sizes (32,10) and (8,400) with different numbers of keys to get LSM trees with different numbers of levels. For (key,value) with sizes (32,10) I used 10M, 100M and 1B keys. For (key,value) with sizes (8,400) I used 1M, 10M and 100M keys.

Results for (key,value) with size (32,10) are https://gist.github.com/mdcallag/19bc721ce59f3a8430c56a703491e5da

Results for (key,value) with size (8,400) are https://gist.github.com/mdcallag/4634824ce75dabf2cc13eb334db65451

For each configuration the db_bench readrandom test was repeated 3 times and the table lists the median response time.

### Steps to reproduce the behavior

The main test script is https://gist.github.com/mdcallag/060e773893450ab6518ab97fd437833c

Helper files and scripts are at https://gist.github.com/mdcallag/e7e1c905026ff8e845c113f02169403c