That seems like a weird reason.
You can lazy load your route implementations
You can lazy load your route implementations
I've never attempted lazy loading inside a Faas platform but I think generally you would be better off creating small deployable chunks and letting the platform handle scaling them up / reusing as necessary. Less fighting against the platform and if you have to maintain strict boundaries to enable lazy loading only the needed code for each route it's a short step to deploying those as their own functions anyway.
I don't know what PaaS you've used, but at least with Google Cloud Functions, you can do a minimum number of instances. Set it to 1 and you never have cold start issues.
There is a middle ground that will be much faster and cheaper all around.