Did you get the Temp upgrades to expand the gauge? The only way to increase her temperature is getting pink hearts, which appear very randomly. So I'd say that's where the spawn speed and max spawn upgrades are good. The faster you clear the hearts, the faster you'll find a pink heart, and if the limit is higher you'll have more chances of spawning one. The temperature side upgrades are also great to quickly stack up points, the phone gives x2 spawn speed and vibe gives x2 points. The Slime is an auto-pick up upgrade. Using all three at once allows you to quickly fill up the lust bar