Performance Issue when upgrading 12.1 to 18.5

After upgrading our test-Environment from 12.1. to 18.5 we have some Performance issuses:

One Test-Batch do every time the same things. In tkprof-Output we see that the cpu-usage is higher at 18.5-Database. The fact is reproducible.

In another case we see, that the phyical read requests in 18.5 is very much higher than in 12.1 (by the same select).

Could this be the problem and how can we influence the read requests?

Here are some tkprof-Outputs:

Runs in 18.5:

OVERALL TOTALS FOR ALL NON-RECURSIVE STATEMENTS

call     count       cpu    elapsed       disk      query    current        rows
------- ------  -------- ---------- ---------- ---------- ----------  ----------
Parse        5      0.00       0.00          0          0          0           0
Execute 394086     28.73      56.80          0          0          0           0
Fetch   433103     18.39      89.08      36687    1816571          0     1048992
------- ------  -------- ---------- ---------- ---------- ----------  ----------