As you change your app, Bubble does calculations behind-the-scenes to help optimize page load times in the future. One consequence of this is that the first time an app is visited in runmode after a lot of new changes have been made, that first load may be a bit slower as Bubble does its calculations. This slowness on the first load should not happen again after the calculation is done (which usually takes at most a couple of minutes), until of course the next time major changes are made. Note that this calculation happens separately for your development vs. live versions, so if you aren't deploying to live frequently, this effect may be more noticeable there.