Normal.
For the RENIX 4.0 (1987-1990,) there are two key sensors in play - the CPS (Crankshaft Position Sensor) and the SYNC (Camshaft Position Sensor, this one is in the distributor.)
I've read a few reasons myself, but there are two that stick out in my mind...
1) The ECU won't put out any pulses to fire the coil or fuel injectors until it reads 300 crankshaft RPM (probably apocryphal, don't know where I read that one.)
2) The ECU waits until it gets a valid signal from BOTH the CPS and SYNC sensors before it fires anything else. This one makes a little more sense - but the RENIX ECU is able to "guess" when it can't get a SYNC input (takes a while - I've tried!) and get things running anyhow.
If you suspect your SYNC sensor, unplug the connector at the distributor and see if it changes anything. The distributor, in the case of the RENIX system, actually has very little to do with ignition timing - I've been able to turn it 180* EACH WAY with the engine RUNNING, and it won't even skip or stumble...
As far as the other reason (300 RPM,) all I can suggest to you is you make sure to stay on top of your battery. I've replaced my battery with a Red Top, I've got a high-output alternator in place, and I've replaced all the mains leads with heavier - and cleaner - wiring, and my 88 now starts fairly quickly (3-5 cranks, I believe.) If you'd like information on the wiring or the alternator, check the link in my sig - Rod (San Jose Generator) will ship, and I've sent stuff all over...
5-90