If the only problem you have with header-only programming is compile times, might I humbly suggest that the problem you are actually concerned about is entirely

If the only problem you have with header-only programming is compile times, might I humbly suggest that the problem you are actually concerned about is entirely somewhere else (hint: most language compilers are terribly unsmart about what changes trigger recompiles, as the cost of compiling the entire source tree isn't too terrible).