processInstance start time increases with the increase of variables!
is why?
What factor are we talking? On a full database or on one which is just filling up?
I could think of several aspect such as increased transmission times of data structures, time to de/serialize objects, more data and more rows (ACT_RU_VARIABLE) to be written, many database related configuration and tuning parameters,…
To discuss this in more debt a lot of detail on your test case would be required.