That's a difficult one. I'm not even sure that a single table with 18 expressions is even a good idea, let alone 5 of them. It all depends on the amount of data you put into these expressions. For example, the effect of splitting one object with 90 expressions in 5 with 18 expressions each may be minimal if they only process a few hundred pre-aggregated rows.
The best way to get an idea of the perfomance difference when handling your full set of records is to do a test: 1 table with 36 expressions vs. two objects with 18 expressions each. You can use the Calc time & Memory columns on the Sheet Properties->Objects tab to get an idea about performance. I'm more inclined towards the latter...
If you really want to improve speed, I recommend you read this interesting document about QlikView optimization.