Without metrics, it’s hard to say where the bottleneck is for your application.
If you temporarily upgrade to a paid plan you’d be able to take a look at your Memory and CPU consumption to get a good idea of what might be going on there. If you do that, snap a screenshot of your metrics before reverting back to the free plan.
It is worth noting that Heroku and Render are architected in different ways that don’t exactly make it an apples-to-apples comparison when comparing plans.